Right now I'm working out a design for torque plates to replace the clamping dropouts on the rear of the SB Cruiser trike, and allow me to put my hubmotor wheels in from the side, instead of from underneath, which will make it much easier for me to work on the wheels or tires on those occasions I do need to, since I won't have to lift or tip the trike to do it, since my back/hip is still healing but will probably never be like it was.
Basically it's just about a 2.5" square steel plate, at least 1/4" thick (thicker if I can do it, or doubling up the plates, for more axle-flat-to-hole-flat interface), with an axle hole in the center a lot like what you have there, but with disc-caliper mounting tabs to isntall Avid BB7's back there like I have in front (since I can't rely on regen brakign in all situations especially with the Grinfineon controllers (since if a hall goes out they switch to a mode that has no braking).
Kinda looks like this, but probably with the disc tabs on the right edge instead of top:
There will be four plates, two of which will be outboard and welded to a section of frame that's removable, bolted to the rest of the trike frame, highlighted in green. The disc brakes are mounted there, under the lower "rail", with the rightside one forward of the wheel, and the leftside one rearward of the wheel, so that the calipers can be the same (already have them) and so can the mounting plates. The inboard plates will be welded to the main trike frame, in place of the clamping dropouts that are there now. They will still have disc tabs because it's easier to have them all made the same, most likely.
I have to setup a motor, rotor, caliper, etc., in a picture so I can then trace that out and measure stuff and build it in a CAD program, so I can get these made. (I planned to do them by hand, but worked out that it will probably take me weeks, at least, to do them myself, with the time and energy I have available).