There exists a parallel port driver for Canon LBP-800 and a USB port driver for Canon LBP-810. Now, I have modified the parallel port driver for LBP-800 so it works with LBP-810. Hopefully, it still works with LBP-800. That remains to be tested, though.
Download from http://berges.dnsalias.com/~sveinb/lbp800-0.1.2.mod.tgz
Known problem: The driver hangs sometimes. A hung driver is a process which can be killed easily.