So, over time I've had these RX-7 calipers and a bunch of 240 junk and scratched my head to make it work.
Here is what I've come up with for me as a package:
1st problem is the split:
The rx-7 calipers only provide for one line, so your choices are single diagonal or front/rear. After doing some poking around, I discovered that the ABS 240s use a front/rear split with only one line to each of the front calipers stock and one line to the rear axle with a T. FI was lucky to find an ABS central california car in the JY, which is really rare, since most junk is really old and beat to hell and rusty here. SCORE! I couldn't believe how easy the brake lines came apart. Like no heat, pb or anything and a simple open end wrench was plenty. I hate them and their lack of any crust sooo much. The brake lines on my 700 are practically welded together.
Anyway, I'd also like to add a hydraulic hand brake, and the diagonal split isn't a go for that and complicates that. I took the entire ABS braking system including computer and ABS struts out of a 93 245, since I also like ABS, at least for being able to lazily steer on slippery stuff in the winter.
Pics tomorrow as I finish bolting everything in. In the front/rear setup, the stupid junction block is elimated, as are the rear restriction contraptions. All that is there is a front/rear master, basic junction block with the "brake failure" light just being a fluid level sensor in the fluid reservoir on the MC.

Next up, choices for the MC:
For the MC, I am using the abs front/rear MC since I have it and it works. I also want to keep my brake booster, but just change it to the skinny style so my 240 isn't so much like an RV like ones with the big booster can be. with little modulation.

Lines: The stock ABS lines and my IPD lines seem to work just fine with the RX-7 junk. I need to go to the dealer and grab another rear axle T piece to plumb in my hydraulic hand brake made out of a 740 clutch master.

Rotors: By drilling all 4 holes in a line on the squared up adapter stock, one can get basically a perfect fit for the large 740 rotors, though rotor width is unfortunately restricted to 22ish MM, which is a little skinny. Oddly enough, the "big" 740 rotors seem to scoop air from the outside to the inside. Goofy for sure. Glad my dust shields live in the trash..though my ABS sensors may object. Anyone know if there are a different style of "big" 740 rotor that pull air from the center out like a sane car with brake ducts and such?

Wheels: The 740 steelies seem to fit, virgos need spacers, but don't fit at all, the MSW multi-x fits and the 780 multi-x wheels fit, which works for me since I have 740 beater steelies, 940 adheras and the multi-x, which all seem to fit.

ABS: The rear diff tone ring will need to be added, but the 240 abs has its own harness and is pretty simple to take apart. Same basic ABS is a 700 really. Not so great, but I couldn't outbrake my 940 abs on icy snow all that often and I can really only brake or steer, not brake AND steer. On gravel or dry dry pavement, I own the ABS almost all the time. Good thing the 240 gods invented dash switches!

Struts: The st00pid abs struts use different spindles which are larger diameter. Larger diameter is nice for someone like me who will likely jump the car a few times. Not so nice in that wheelbearings are rare and no one but the dealer seems to carry them. They are notably larger than the non-abs large spindle 240 struts though, which is nice for being a savage to your car.

Piston sizing: The RX-7 calipers use the same or very similar piston sizing to my 240 calipers. Hopefully overall balance and mechanical advantage will be more or less the same with just better heat dissipation. Anyone know what the different sizes of 240 rear caliper are to help me adjust the balance a bit since I lack an adjustable balance bar and wish to keep a brake booster on this car?

Anyway, my car is coming together. The ABS struts are in with fresh mounts and it is getting power steering *yay* with a fresh almost new 05 rack from a 93. Currently it is using stock 240 abs calipers and rotors and junk just because my heap of 740 rotors is far away.
Anyway, pics to follow of my +abs+ RX-7 contraption!