This arrangement has worked out pretty well. Details below for those interested.
I found the Universal Inflatable In Seat Lumbar Air Bladder on eBay. This one seemed to span the width of the seat the best of those available, at 9" x 5" (actually 8-3/4 x 4-3/4).
I put some cloth on the outside of the bladder for protection, and outside of that some 1/32"-ish flat plastic I cut out of a storage box for even more protection and to better distribute the bladder pressure. That pressure distribution would be a user-preference thing; some might not want it at all. There is the stock bar across the lumbar location that is actually more of a hindrance than a help.
The bladder assembly isn't mounted to anything. I just let the seat back hold it in place. It might not be 100% stable that way (I haven't noticed anything), but it's easy to get to for relocation if desired.
