actually... in my compound days i remember people would use what they called a draw table. basicly the bow would be secured to the table, and then drawn by a cord same as on a tillering tree, but there was a weighted pencil attached at the d-loop (where a mechanical release attaches).  and then you would tweak the cables and/or tiller until it would draw a straight, perpendicular line.  that might be something we can adapt to our purposes. whatchya think?

That is a very interesting idea Eric.  Prototype something.