A variety of low-cost "embedded" machines support Ethernet and USB but have no video output (e.g., Kuro Box, NSLU2, Soekris). Meanwhile, there are a variety of USB VGA adaptors in the $80 range (e.g., Tritton). The project would entail writing a /dev/draw driver for one of these devices (your proposal should include evidence that there is enough publicly available information on your selected device that you will be able to write the driver).

Note that on Plan 9 a USB device driver is typically a user-space program (i.e., this project will not require kernel development).