It's possible and pretty easy too. Just make new mode and take default from x240 resolutions for example. Then just change vertical resolution to 200. Then tune with pixel clock to wanted frequency.
Has always worked fine with CRT monitors, but some TFT monitors can be picky with frequencies or simply don't work at all with low resolutions.
You can also make x256 and x512 modes for some more compatibility. And for example 480x360 etc for some games, which are too slow for higher res and too ugly for lower res
Like Napalm with Zorro2 gfxboard (for faster bus it's also fast enough for bigger res).
