Thing is even moderately high resolutions in SW will pwn slow processors. And let's face it, most computers running SW Quake out of necessity (i.e: no 3D acceleration at all) are unlikely to be running on anything much faster than a P2.
I don't think there's as much of a demand for high resolutions in SW Quake* as there is for being able to play at 320x200 on a £20 laptop.
*There may be a demand for easily reverting to the feel of SW Quake, e.g. turn off all the eye candy with one option (that includes all texture filtering).