Dustin-- I've been mulling over what you recommended, and also used my cradle this weekend. it was good enough and I wouldn't have thought about the ball swivel if it hadn't been for your post... but alas, I had it in mind while using the cradle and could definitely imagine how it would have been useful.

So I have a couple of new ideas:
1. I have an old heavy duty tripod ball head (slik ball head 800) that I rarely use for photography. So I put a threaded insert into the edge of my bench to mount it. I ordered the heavy duty (3/8-16) base plate for $20 and will be picking up a coupling nut from Lowe's later today. I'll attach the nut to the plate then use a long bolt through the center slot of the cradle to make a sliding attachment to the ball head. since there is not a lot of force used in checkering, I'm thinking the setup will be stout enough, though not as stout as your power arm (of course, this cost me a fraction of what a power arm would cost)

[Linked Image from i.ibb.co]

2. I saw a post from the furniture maker Peter Follansbee this morning about an alternative to a shave horse for draw knife work. The design is by J Alexander and involves a modified pipe clamp. It is a modern version of the wheelwrights fiddle. I think this idea could be used to create a cheap, lightweight cradle that could be rotated along the z axis as well as horizontal and vertical. I'm thinking about ways that it could be held securely to the bench.

[Linked Image from i.ibb.co]

Jim