yes you cn use 27c256 with 12 volts just fine. I do near eveyday.
You might want to read this post http://www.willem.org/cgi-bin/yabb2/YaBB.pl?num=1144884107 regarding reliability. Programming with out-of-spec VPP isn't as bad as not setting VCC correctly, but you shouldn't make the mistake of assuming that programming and verifying in your programmer is the same as programming the device reliably...
The amount of charge stored on the floating gate is affected by both the programming VCC (affects the number of electrons in the channel that can be deposited on the floating gate) and VPP (helps attract electrons from the channel).
If you program at low VPP and incorrect VCC (which most with Willem programmers do), you're affecting program margin and that can mean reduced reliability.