I've done one
http://www.circuitbenders.co.uk/newsarchive/DR550.html although in hindsight i probably wouldn't use a patchbay if i did another one. Most of the effects seem to tail off and fade away after a quick connection, so they are probably more suited to switches or even pushbuttons.
You'd probably need some schematics to figure out if a lithium battery backup was possible. Theres definately some discussion of how to do this to do with another drum machine somewhere else on this forum, so that might be a starting point. The main issue would be to stop the DR550 trying to run from the backup battery and eating it in a few hours.
Independant outs, tuning and midi outs won't be possible unless you fancy rewriting the firmware, which you really don't
My notes show a diagram of a chip labelled Roland, and say i used pins 2, 3, 5, 7, 8, 9, 10, 11, 12, 14, 15, 17, 18, 19, 20, 22, 23, 25, 26, 28, 29 and 31 on that chip. Hopefully thats with the standard pin numbering.