It appears possible, though probably not as easy as you'd like or in the way you expect. The display system on the Internet tablets isn't designed for mirroring or extending the built-in display.

That said, the first result to a Google search for "linux targus acp50us" shows that the VGA chipset in your USB dock can work with Linux and even provides information on the small source code modification that's necessary. Compiling the kernel module for your tablet will likely be the stumbling block there.

Lying beyond getting the kernel to recognize the USB-to-VGA chipset is the issue of doing something with it once it's working. aflegg has previously posted a link to an IRC log showing that dragorn on #maemo got an external display to show static images. And to the best of my knowledge, that's the most anyone's done thus far.