I discovered a short time ago that Calamity (
he's behind GroovyMame among other things) made an
experimental patch for the English version of WinX68KSpeed. Scroll down the thread for a detailed English explaination, or download the archive there right away, there's also one inside.
It basically allows the emulator to switch into native resolution modes in Fullscreen if you have the proper video modes installed (with WinModeLines or other tools). I did some quick testing on my SDTV as well as on my standard CRT (31 kHz minimum) monitor. Crashes aren't fatal, you can simply restart the emulator if the needed display mode isn't found.
The modelines listed will of course have to be tweaked depending on your setup. E.g. to use a 256p low res mode on my 31 kHz CRT monitor, I doubled the refresh rate (this results in a "true" scanlined look. You can try it in other emulators).
On my TV, I found that the 256/512 line modes are problematic with the given rate (55 Hz), it looks more reasonable at around 53.
From the post it's clear that Calamity didn't know if the refresh rate is actually constant through the modes, does anyone know more about it? The 512 line modes afaik indeed run at that refresh, but the others?