I... I don't know if it's possible without a romhack of agbfirm itself...
I mean, for actual emulated games we have a configuration file that's undocumented, but when you see "Width=number" it's a dead giveaway, on the other hand the regular VC emulator is multiplatform.
In GBA mode nobody would have thought disabling the screen filter would be so easy until it was actually figured out; but this doesn't necessarily mean the scaling can be forced, especially given this is virtualization with very limited background processing!
