BC supports "custom" resolutions at an extend, one is that you need to be running in -TestMode, and second, you need to manually edit your options.cfg and I suppose don't enter the Graphics menu.
And I think, that you don't have the anti aliassing mod active (if you have it).
Oh, another problem, your GUI will look like it's in 640x480 mode (limitation of the scripts).
Lastly, this has gotten mixed results, only 1 known case where it worked.
Now, also check this thread out:
http://bc-central.net/forums/index.php?topic=3184.0[EDIT] The anti aliasing mod is called "3rd Era Jaggie Fix" and can be found in the Startup options of UMM.