The H.264 encoder algorithm is, of course, unaffected by temperature. It's an algorithm.
There are many embedded devices that perform H.264 encoding, and these *will* have temperature limits for their nominal operation. What sort of budget/mass/envelope are you considering? I presume that you intend to encode video to H.264 in a space-based project?
Any details about the orbit will be very useful to know as radiation hardening might be the limiting factor for a commercial device.