One of the developers for emc participated in engineering week. One of the competitions was a rat trap powered vehicle. To get the most out of the spring - he made what would be called a fusee. A normal encoder on the spindle A,B,Z - (you need one with an index pulse). Emc will blend multiple spindle synced motions together. He said there was 26 G33 moves in a row.

You can read about it here
http://jmkasunich.com/cgi-bin/blosxom
http://jmkasunich.com/cgi-bin/blosxom/shoptask/fusee-1.html <- not finished yet..

YouTube - Variable Diameter/Variable Pitch ThreadCutting with EMC2

http://jmkasunich.com/pics/DSC00563.JPG