I think I have the frame figured out; it's just fit together and not welded or anything, in case I or someone else posting here figures out some stupid mistake I've made.
The couch-futon frame mentioned above:
The bike frame currently looks like this:
The Trek chainstays are spread out, by straightening them at the former bends just at where the tire would've been. This was very difficult, as they're quite strong.
Had to put one foot on the seattube, and the other on the chainstay right at the bend, grab the dropout with both hands, and slowly pull until it was straight. I'm sure it's not great for the metal, but if I have a problem with it I'll replace the whole stay with some other tube. I just don't want to unless I have to.
I couldn't bend them downward, though, to make them line up with the BMX stays. So...I adjusted the BMX stays so that they'd point to where the Trek seatstays I'd removed before would be able to complete their curve (from the seatpost end) right onto the Trek chainstays. It worked more perfectly than I would have hoped for.
Then the vertical diagonal is black tubing from the couch-futon frame, running from the highest point of the bend up to the juncture of toptube and seattube on the BMX frame. Another piece of that black tubing is fishmouthed and tapped between those elbows of the bottom frame, to triangulate it and help stiffen the whole thing, as well as to prevent it moving inward or outward under loads.
I still would like to make it level between the Trek BB and the BMX bottom of the dropouts, but to do that I would have to cut the Trek chainstays from the Trek BB, at least at the top of them, then reweld them at a different (lower) angle, filling the wedge-gap with other tubing. (or cut the wedge from the bottom side, and reweld the cut edges together, but they may not be long enough if I do that; they barely are now, overlapping the rearward tubes by only 1/2" or so).
The brake studs and probably the accessory hardpoints will be removed from the former seatstays after I am sure how the frame will be done. Then the studs will probably be welded to the front fork's bridge so I can have both rim and disc brakes, which will probably be run to the double-brake handle AussieJester sent me. Then I can engage both brakes simultaneously, adjusted so they both take some of the load, and I should be able to lockup the wheel no matter what kind of load I have on the bike.
An alternate for the vertical black tube to the seatpost/toptube juncture would be to run it to the toptube/seatstay juncture instead.
I'd also like to add something from the same elbow point of the bottom frame to the BB of the BMX, to stiffen to that point, but it may be unnecessary, and indeed in the way of other things.
I'd actually kind of like to remove the entire fat BMX downtube, but I think it is needed for stiffness in the resulting bike. I'm just not sure. I know when I removed the downtube of the rear frame of CrazyBike2, to make more room in the central space, it made the bike significantly less stiff, but it was not shaped or fitted together the same way as this one.
The BB itself (a one-piece-crank type)is not usable directly, but it may work to hold a threaded BB inside it, to run Thud's jackshaft in. A possible setup of this is shown below, but it wouldn't have the jackshaft so far out to the right in reality.
In this, as in CrazyBIke2's powerchair drivetrain, the pedal chainrings are on the left, as is the motor output. Both feed into the jackshaft via freewheels installed on it. The output of the jackshaft feeds back to the rear wheel's 3spd IGH. Sprockets used were just handy and are not likely to be the actual ones used in the drivetrain.
The motor shown is the 9C/GM hybrid, but the Fusin could be used there just as well.
The chainline should work out withotu any interference to the frame and such, as far as I can tell using these loose chain strips.
Another shot of the drivetrain area:
Since I might need to seriously gear up the motor for the race, I needed to make sure that the 144T #25 sprocket will fit in the space if bolted to the side of the 9C/GM:
It does, just barely. If the jackshaft is placed elsewhere or the BMX downtube/seattube are removed, then it will easily fit and clear everything.
Of course, if I gear it up that much, I'll probably have to pedal a lot just to get started up without blowing stuff up from overcurrent.
That 144T is pretty big:
but it will still easily clear the ground, too, even at full droop/bump of the suspension.
Note that the cylinder currently in place of the shock is not permanent, just there to hold the frame at the right angles and such. That cylinder is an adjustable locked-out gas spring, which means that it isn't actually a shock/spring, but rather is a position adjuster, by pressing the release pin at the tip of the extended shaft. On the powerchair this is from, there is a lever like a brake lever that remotely engages this pin, so one may adjust the ride height/angle of the powerchair.
I may well use this cylinder for that function so I can fully lock-out the suspension or limit it's travel under heavy loads, etc. Depends on what I end up with for a suspension, since that still isn't decided.