Re: BenQ PE5120 and 854x480
The bug report you attached against the 'nv' driver also shows that 854x480 fails to work:
(II) NV(0): Not using mode "854x480/75Hz" (no mode of this name)
(--) NV(0): Virtual size is 1024x576 (pitch 1024)
I'd suggest talking to BenQ at this point, as either their documentation is erroneous, or possibly something is wrong with your projector. Perhaps they can provide you with a modeline for 480P.