I thought there was a more detailed walk through, but this is the the only mention of the dirty fix that I could find right now:
http://www.bc-central.net/forums/index.php/topic,3184.msg57768.html#msg57768Make sure that you have all options set before you try that, as bc will revert back to a lower resolution upon entering the options (or display?) menu afterwards.
KM also has some more custom resolutions included, if you use that mod - though then you can't play the campaign.
There is also a more elaborate way to enable other resolutions, that requires that you have the sdk, and edit a couple of lines in *some files i cannot remember*
I know someone else tried that recently-ish, so you might be able to find more info about that here or on the discord server, if you search.