Well, if the Qualcomm-based GS4s supports HDR video on stock (only certain camera sensors support it), it should be possible for Qualcomm variants. Samsung often deviates somewhat from Qualcomm's reference HALs, but rarely significantly for Qualcomm-based devices.
Unlike stills, there's no way to hack in "software HDR" for video like CM was able to do with older devices. If the hardware doesn't support HDR video, it isn't going to be supported. That basically means devices with the Sony IMX135 sensor (Xperia Z/ZL, Find5, and I think GS4) and (apparently) the HTC One's sensor right now to my knowledge.
Enabling this for Find5 could take some time - we have a major kernel rebase effort to finish first.