The KLM agents were unwilling to allow me to check in without buying a return ticket. I did some quick shopping around and, not finding anything I liked after a quick look, bought a refundable ticket before my 30 minutes of free WiFi ran out. Turns out that this was overkill; the agent only asked for a screenshot, and he asked me to email it to him. Not only that, he didn't actually check his email before giving me the boarding pass!
As expected, the passport agent in Amsterdam did not actually ask to see my return ticket. He did, annoyingly, ask about where my exit stamp from my last visit to France was, but at least this time he was easily satisfied when I told him I never got one. (The passport control in Charles de Gaule was of the "this closed document looks vaguely like a passport—go ahead" variety.)
ETA: I got an apology from @KLM: https://twitter.com/KLM/status/489682946865905665