And again...
Drawing this thing up yesterday I was just going with the flow of ideas, and took no time to do any load calculations. Turns out that the unsupported base tube leading to the rear assembly pivot point just wasn't gonna cut the mustard, so I did some figuring today, and modified it a bit. With the seat tube brace extended to intersect the bottom rail, and moving the mid-drive mounting point back much closer to the pivot point, it works.
I also decided it needed some stiffening of the rear triangle as well. The basic lines of the bike remain essentially the same, with short-travel front BMX type shock forks. I'm considering what type of handlebars too give it. Suggestions?