I have bought in the end a Plugable UGA-2K-A USB 2.0 and have done some tests, so I would like to share my experience with the module on the ARM9 module.
By using udlfb driver and framebuffer I could run a QT Embedded application on the AT91SAM9G20 module. The tested resolution was 1024x768. Here are the results:
drapdroprobot, collidingmice - were a bit slow (I guess around 20-23 fps with approx. 80% CPU load)
elasticnodes, diagramscene, anchorlayout, other layouts/dialogs - were pretty usable (I guess around 23-27 fps when smaller amount of graphics were changed - 1/4 of the 1024x768 orso). The CPU load was high when big part of the buffer was changed (enlarge of window etc.).
320x200 MPEG video using mplayer - around 23-27 fps
The diagramscene and other layouts/dialogs have very good response. It is fast enough to not annoy the user. I think the speed was comparable to a AT91SAM9263 module with TFT running at 200 MHz and 640x480 resolution, but with the USB-VGA adapter on AT91SAM9G20 I could run the same applications at 1024x768. It could be well used for simple dialogs applications in my opinion. I am not sure how much of the load was rendering and how much were USB communication. But I would expect that with USB High Speed it should be visibly better.
Kind regards, TDK