robtillaart:
sounds feasible, you need at least 3 positioning motors, and quite a bit of (gonio) math to get the movements right
I would start with getting one motor operational. and if that is smooth add a second dimension etc
Thanks Rob - some good search words to start from there 
Not least 'gonio' ... I had thought briefly about the math / logic involved, but was mostly putting that off until (much) later. I thought a slightly random algorithm, with a preference to continuing in the same direction as current motion (in all dimensions) until reaching an 'edge' would be a good place to start.
I'm totally new to motors - I thought stepper motors wouldn't be appropriate here (partly the range of motion, partly the jagged nature) ... could you recommend even just an example, say on ebay, of a positional motor that you think would be appropriate to this type of motion, and distance of travel? There'd be close to no force against its movement in any given direction, which I had hoped would mean I can get away with much quieter, and lower-powered motors.
thanks again,
Jedd.