I think that beta firmware is a valid firmware.
(...)
because it is hard to find a camera with that firmware
Cams with prerelease fw certainly exist, but they are too rare and you don't normally get them from official sources. In this case, you can expect approximately a single user. Of course, there's no problem making such a port, it's just that the chance of getting feedback is much lower.
Note, however, that you must make a correct bin_comp_fix.h (by hand) or switch off the compatibility check (not recommended). The script that generates bin_compat.h is hardcoded for GM prefixed firmwares (it works for more than 98% of all ports).
