Can a WS 6.0 virtual machine be booted off a USB 1.1 and/or 2.0 flash drive that is attached to a physical machines usb port?
No. You can attach a USB device to the VM but it does not appear as a boot device in the VM BIOS.
But it can be booted from a custom made DVD ISO. Which you could place on USB stick...
You mean you could make a DVD image, put it on a stick, and have that be the file that is the drive image for WS?
No, just bootable ISO. Or you may try to put whole guest (including virtual drives) on USB stick.