Alrighty then! Further to my previous posts, I have been doing a bit of digging.
1) GarageBand has a plugin called 'Vocal Transformer' - it is a very basic pitchshifter, that raises tone and/or pitch (but 'digitises' the signal quite badly!) If you want something to sound very 'computer' then you've got it!
2) You CAN access non-Logic AU plugins in GarageBand. In one of the two plugin slots on a Real Instrument track, if you go to select a plugin (as above) and scroll right down, you will find any AU plugins that are resident on your system. I have a few Cycling 74 ones & also NI Reaktor FX cropping up in there; they all seem to work just fine!
Sweet Jebus. Considering how basic a program GarageBand is, it's got a handfull of REALLY nice 'Pro' features that just need a little rooting out. The audio 'edit' window is pretty sexy, too.
If I find anything else that might help, I'll let you know!