I just read an article about this. The article had a link to the letter of the law in California: http://www.dmv.ca.gov/pubs/vctop/d12/vc27602.htm
There's a few arguments you can make. Technically the Google Glass screen is not a "television receiver", a "video monitor", a "television" nor really a "video screen" exactly. It is something technically different than all those. They might be able to argue it's an "other similar means of visually displaying a television broadcast or video signal that produces entertainment or business applications", but then you weren't displaying a TV broadcast nor a "video signal" at the time, right? I don't think looking at system menus or whatever you normally see through the Google Glass screen other than actual "videos" can be classified as a "video signal". They should have to prove that you were watching an actual video at the time, you can ask the officer what they observed you looking at through your Google Glass at the time. If they can't say what they observed you looking at through the Google Glass, then where's their evidence?
The article said your Google Glass wasn't even on at the time, how can the officer prove it was on? Did the officer see something on the screen? If so what was it then? How can the officer say for sure it was a "Video signal" oppose to something else? If the officer can't say what they observed you looking at, they got nothing. This is no different than having an actual TV screen or monitor in front of you, but it being turned off the whole time....that's perfectly legal because it's not on.
And then there's exceptions for GPS and Mapping displays in the letter of the law. This is also a good argument on your part. Your Google Glass can do those, so your Google Glass should fit the exemption.
If you're going to fight this, this is what I would do in court to prove your case. I would ask the officer to explain exactly what law you're accused of breaking and for the officer to explain exactly what the law is. I would ask the officer what a "video monitor" and "television" is. Drag it out to the basics if you have to "So a TV is for watching TV signals or DVD's right? A video MONITOR is for a home computer right? What do you mean video screen....to watch videos?". Then I would ask the officer if there was any exemptions to this law. Then I would ask the officer to tell the court what they think Google Glasses is, and if they're aware it has GPS and Mapping capabilities if they don't mention it. (Obviously they're going to play stupid and describe it in a way to fit violation of the law and omit anything that fits the exemptions). After that ask them what exactly they saw you doing that was in violation of the law, get them to say specifics as to what they observed was on your Google Glass screen to indicate it was in violation or to indicate it was on even. The officer is going to try to be vague as possible to protect their case so you're going to have to drag it out of them, it's tedious but necessary to get it on record in court.
I would then produce into evidence a print out of the exact wording of the law they've accused you of breaking, have the words "television receiver", "video monitor", "television", "video screen", "television broadcast", and "video signal" highlighted, also highlight the exemptions. Then I would read out the law for the court word for word. Then I would explain how the Google Glass screen are none of those by producing print out of definitions of what those other things are (Use Wikipedia articles or something that can look legitimate or accepted and can be entered into evidence). Explain what each of those are and how Google Glass are not them. Then produce print outs of articles and/or photocopies of the Google Glass manual which describes exactly what it is, explain it and enter it into evidence. Make sure you highlight the GPS and Mapping capabilities as listed in the evidence. And then lastly I would describe (using printout evidence if possible) how somebody can or cannot tell if Google Glass is on or not while not wearing the device. If there's no way to tell, then the officer couldn't possibly know and have failed to prove it was on. If there is a way to tell, and the officer did not describe the such way correctly (how could he if it was actually off) then again the officer failed to prove it.
Finally I would prove your case by declaring that the Google Glass does not fit any of the violations of "TV screen", "Video Monitor", and such, and it fits the listed exemptions of GPS & Mapping, and the officer failed to prove the device was even on by being unable to describe what indications he observed that would suggest it was on.....and that you have proven beyond a shadow of a doubt that you were not in violation of said law.
There's a few things to look out for too, in my area it's common practice for the officers to try to scare people out of fighting just before trail. They would approach the accused in court and tell them they're going to lose but if they plead guilty they'll reduced the fine. Don't fall for this bully tactic, don't even speak to the officer before trail. Sometimes they'll do this tactic and if the accused doesn't fall for it and wants to still fight, the officer will immediately tell the judge that they're not going to pursue the charges when the trail begins...because the officer knows they're going to lose or look bad when the accused tells their side of the story of the case. For you, your case might not even make it to court since you're getting so much publicity. The police might decide to drop it as soon as you file to fight it in court because it may be bad PR for them. The officer might decide not to show up to court either, letting you win by default. Something to think about. Also be aware that the police might be monitoring this thread or anything you might be making public. So don't say anything publicly that is incriminating or may weaken your case, they might bring it up in court as evidence (seriously, not being paranoid, insurance companies do this for accident claims).
Anyways, I hope all that helps. It's a pain in the butt, but if you do the prep work....your chances should be pretty good. Don't be afraid, at the very worse the only thing that could happen is losing the case and having to pay the fine....which is what's going to happen if you don't fight anyways. I hope you'll fight this and win, because this not only affects you but everybody else in the future which will be stopped while driving with Google Glass. This could have big ramifications for Google Glass itself. Looking forward to seeing how this turns out, and hoping you the best.