Several days of playing with motors and opto interrupters (which aren't as fast as I'd like 
) which finally ended up with me deciding that the optical encoder disks on cheap ebay motors are worth buying some more of (throw the motor in the corner as basically useless and use the optical encoder which with enough care can be pried off the motor shaft to be redeployed on more powerful motors.
This approach should work fine in stuff like this which has identical sections of varying size. Either perl or python can parse xml (I expect there is a python xml parser as well as the perl one, but even the normal text tools in either language should work for something as simple as this). I'll admit to thinking that perl/tk might be an alternative to Inkscape for svg creation since I believe tk is vector based (I haven' t actually used it in for years though and may be misremembering) and going from the vectors to xml shouldn't be that big a deal. A lot of the problems I'm having with Inkscape are because it is trying to do way more than I actually need and that makes it complex and painful to use.
Peter Van Epp