I too am dissapointed with my gigabyte 7870 when it comes to the voltage locks.
I understand they did it to reduce costs (and fried RMAs), but i'd happily pay an extra $20+ for full voltage controll

I tried a voltage mod on mine awhile back, but it didn't work.
One thing that bothers me is the locks aren't just for raising, they also apply to lowering, and some "unlocked" versions only allow changing based on bios table ranges (ie: can only raise/lower voltages when certain mclk/gclk conditions are met). Also the fan controll was locked on my gigabyte.
I did some reading around after discovering mine was locked, seems most manufacturers are doing the same. Some sapphire and msi models being the exception I believe.
My cheap diamond 7850 allowed me to change voltages between 1.13-1.19v
It also had fan control, although the rpm reading was always wrong.
I'd love to see a sticky thread with a detailed listing of known unlocked versions.
Also things get further complicated when special proprietary software is needed, or certain "hacks" ...
AMD has the ADL SDK which is designed for changing settings such as voltage, apparently many board makers are using their own closed software instead of supporting this exclusively. This locks you into the software that whatever maker ships, or sitting/waiting for some other software to reverse engineer their nonsense.
I can't help but think that things will only get worse going forward as they implement more complex power/boost features.