It probably is possible, if someone was to decompile the APK files / libraries necessary for face unlock, recode them to use the back camera, zip it back up and load it.
However, there are other things to consider. If I recall correctly, Face Unlock was introduced in Android 4.0. The developers (TDM, thekraven, Bobzhome, etc...) are already working very hard to just get the operating system running as best as it can on our phones. Reading the threads for those various ROMs, it's not difficult to get even a mild sense of the stress they are putting themselves under. Adding implementing Face Unlock to that list will never happen, because honestly, nobody will use it. It's purely a "just because we could" project for a phone without a front facing camera, and nobody who has the knowledge has any time for that, nor should we expect them to try.
So, theoretically, yes, we probably could have it, but the reason why probably isn't technical so much as..."choosing your battles", so to speak.
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