You can get small (about the same footprint as an ATMega, but an inch tall) DC-DC converters that will give you +/-12V from 5V.
I take it you mean this to be a split supply for the opamp? - If so does that mean you're in the single supply opamp won't work camp?
Then you can use an simple inverting op-amp to get a positive voltage, and a voltage divider to scale it down to 0-5V.
I did think about a voltage divider on the output of my inverting opamp for getting the 0 to +ve7 down to 0 to +ve5, but figured a gain of 0.7 would do the same thing. Your thoughts welcome - pros/cons?
From your description, it doesn't sound like that output is very precise or clean. I would add a little noise filtering, and some protection to make sure that the op-amp output doesn't swing above Vcc.
Absolutely right. The output from the plasma unit is notoriously noisy - but other creators of digital THCs claim to be able to resolve to around 0.25V which depending on setup can translate to a fraction of a mm - we'll see what happens. I'd be grateful for your suggestions on filtering. I've had quite a bit of success in my experimentation by filtering with software though - but again your thoughts are welcome.