Mark Spencer did a tutorial explaining a trick he had learned from the book "How to Cheat in Motion" by Patrick Sheffield (which I own. Great book!).
The trick was to make a template for use in Final Cut Pro where you could change a graphic parameter by LINK-ing that parameter to the size parameter of dummy, invisible text. Normally, you can't do that in Final Cut and to change a graphic parameter would require a round-trip to Motion.
I couldn't wait to try it but it didn't work in my case. I wanted to be able to adjust the width of a circle within Final Cut. Sliding the text size slider only affected the thickness of a circle by an imperceptible bit. Turns out that that you have to (in my case) set the Scale higher in the Link parameters while creating the template in Motion to enhance the effect of the text size slider. That's what worked for me.