Does Windows 7 Professional 64bit "XP Mode" support 32-bit drivers? If so, how well?

"No. Drivers installed in XP Mode have no effect on the host system. All XP Mode does is make it so you can run programs in a virtual machine under Windows XP, but have them appear to be native applications on the host system. There is no way to do this with drivers, unfortunately.

If you run all the applications that need the drivers through XP Mode, then yes, that would work."

"The 64 bits driver of my scanner is not provided by the manufacturer nor microsoft

I can confirm you that my USB scanner works fine with its 32 bits driver under XP mode.

USB : This is important as the rest of the seen hardware is emulated (like you video card)
You have 2 Usb ports under XP mode.
Those ports can be linked to Windows 7 usb port/device of your choice even if the device has no driver under 7 64 bits."