Someone did think of it
Thumb operation for often-used keys, that is.
I would dearly love to try one of the keyboards from Kinesis Ergo. They look like the real deal. The Classic Contoured Keyboard (hit the Products tab, then Keyboards, then find the link—these guys need to find a better web designer) does just what I asked for.
I’m just not sure I should liberate three hundred bucks for something that I don’t absolutely positively know will help.
On the other hand (no pun intended), surgery costs more.