Follow-up to this thread...
I decided that the bait board idea that I was entertaining originally was a non-starter for a variety of reasons. I actually settled on a DIY armrest caddy because I want to be able to use it while driving (I do cross-continent solo trips).
The T1N Sprinter offers an advantage in having relatively stiff armrests with flat upper surfaces. This little guy is made of high-grade plywood trimmed in 3/4" aluminum L-stock, and simply held on with three strips of 3/4" Velcro (the two forward strips penetrate the plywood and the one on the "outrigger" simply wraps around the butt end of the armrest). On top of those Velcro strips, added a closed-cell foam pad.
After this pic was taken, I splurged on a brushed stainless steel cup holder insert from West Marine to replace the plastic, so it looks a bit spiffier now.
I can move the items and raise the armrest with this caddy still in place, and walk to the back of the van without having to remove it from the armrest.