I'm late to the party too, but maybe
something like this would work.
I was actually thinking along those same lines, a plate with a spiral pattern, but employing some custom "pegs" that are round (or threaded) on one end for inserting into the "spiral plate", but 3/8" square drive on the other end to accept any socket wrench in your collection.
This way, any radius at any position on the "spiral table" could be accommodated, and you've already got all the "anvils". Except, I guess, when you need the same radius at more than one location.
Remains to be seen, I guess, if the square peg would hold the sockets firmly enough. The more I think about it, the square peg probably wouldn't secure the socket well enough. A 3/8" bolt would though.