That's something I've thought about ever since I got my table. What has stopped me, besides apathy, has been my concern about whether it would look cheap and possibly featured on the fugly tables thread. The examples here make it look plausible.
Is there any bleed through on the speed cloth? Does it take more than one pass to make it opaque?
After reading some of the suggestions on how to accomplish it, I would suggest a simple circle with a opening in the middle as a guide, like a spirograph. Lay the circle on the table, stick the pen in and roll it around the rail. Assuming the rail isn't lumpy. Now that I think about it, it may smudge the ink as the guide slides over it. Maybe lay something in the middle of the table to raise the guide off the surface? Something to think about.