You can use the analog pins as digital also if you haven't done so already.
Another option is to mount an '2560 processor on a standalone board - I posted a link to Schmart boards for one yesterday - and get access to all the IO that is available - might be enough that way; am not sure how many of the 100 pins are tied up for power, ground, xtals, etc. Looks like 11 pins are VCC/GND/Aref, 2 for Crystal, so 87 available IO lines?