Is there any way to manually set the resolution rather than detecting it from the display?

I've got two TripleHead2Go's working great, but the distance between the server and the TH2Gs is a little far. I'd like to put Distribution Amps in those runs. That works fine until I restart the machine. It seems that the DAs don't pass the data from pins 12 and 15 for feedback from the display to the computer. When I click "Detect Displays", I just get a bunch of generic options for resolution, none of which fits the 4:1 ratio that TH2Gs use.

So is there any way to manually set resolutions that don't show up as an option? How about modifying the set of "generic" options to include 3072x768 and 2400x600? I know this is more of a mac issue, but I thought maybe somebody here had experience with this.