I am not sure that the phone being locked has anything to do with firmware - at least for Symbian, if you had a operator branded AND locked device, you could flash a generic firmware with no problems although the lock would still remain active.

I assume that if you have a operator locked device you can still flash the generic firmware without problems. Only thing is that the SIM lock will remain as is, but you don't lose nothing by doing it. Just don't try to downgrade the device after!