I'm pretty sure you can call up GraphTech and custom-order an 8-string setup. (Great Canadian company!)
They'll probably also give you instructions on how to work with their preamp to get it to work with 8 strings, as all synth gear is designed around hexaphonic (6-channel) input.
You're going to want to use a separate amp for the synth stuff. Guitar amps won't do it justice, and that way, you'll also have the option of blending distorted guitar sounds with pristine synth sounds. You can get some pretty decent keyboard monitors (Yorkville makes a few) then run the main synth signal to the PA when you play bigger shows.
For Guitar-to-MIDI convertors, you have a few options:
Product Overview :: AXON :: Roland U.S. - Guitar Synths/MIDI Converters
As far as i know, the Roland GR-20 is the only unit currently on the market that has an integrated synth module. All the other units are just pitch-to-MIDI and you'd have to use either a rack-mounted synth module, a MIDI keyboard or a computer as your sound source. There are a number of older Roland units as well that are compatible with GK pickup input.