Boy do I feel like an idiot, reading some of these responses...
I guess I thought it was a driver issue because, to get Windows displaying on a rotated monitor, the driver has to support it. So, I thought that for rotated MaLa / MAME, it still came down to the video card driver in Windows... when, as people say, its a function of the emu itself...
Anyway, good to know it can do what I want... now I can start investigating how and if I'd manage to have a rotating monitor in a lowboy...