I use a scandoubler in my A4000 which is hooked up to an AOC 17" TFT.
System friendly games work fine, but for 50Hz PAL games it's no good. The funny thing is the monitor displays the picture fine but I get a small warning "floating" around the screen stating that the mode isn't supported.
I've since come up with two solutions:
1) Use WHDLoad (which I do) and force the game to use NTSC, which is 60Hz and will display on most (if not all) TFT monitors. Most games work, some don't.
2) Buy a ViewSonic TFT monitor. They sync down to 50Hz and should display all those 50Hz PAL games. I haven't got any idea if it displays 320x200 sized resolutions but it should. No guarantees on this one though.