The issue is that you want the
NC ABS to work? For this you will need to know what the inputs are - power, wheel speed (ABS sensors), handbrake probably, an output to a warning light, maybe more. There may also be internal programming that requires 'live' engine data (say engine speed related to wheel speed, maybe windscreen wipers, who knows ...) I just don't know, because the ABS uses the engine computer, all that engine and body data is available to the ABS if the Mazda programmers wanted to use it. If you want to just cut the wires to the ABS from the ECU, provided you have a wiring diagram and some understanding of what signal characteristics the ABS needs to see (not just where the signal comes from), that may be doable, but I would not like to be the one pioneering it!
The stand-alone ABS is much simpler, you know what is going into it, feed it that and watch it work.
Back in the day, parallel engine management was a thing. Aftermarket ECUs were pretty crude and lacked the ability to do everything that the factory ECU could do, so the aftermarket computers were wired in parallel to the factory ECU. The factory ECU thought it was running the engine, so everything worked as the factory ECU thought it should, but the aftermarket ECU was actually doing the ignition and fuel so the performance was happening, just out of sight to the factory ECU.
This may offer some possibilities, but with the NC computer being so highly capable (especially in heuristic mode) it may be too smart to operate when it 'knows' things it is supposed to be managing are not responding. The new engine being a six rather than a four, and having the associated fuel and air differences, only adds another layer of complication and difficulty if you wanted to go down that path.
I can only reiterate, if you want ABS a stand-alone system seems the way to go. It doesn't have to be Mazda, as long as you can get a wiring diagram for it you can make it work.
The only other road you can explore is the ECU programming. There is a DIY tablet based system from the USA that offers access to the NC ECU. This is intended for engine management purposes, but it may also offer some insights into the ABS operation, and the extent to which it can be made to work without engine data. Try the big board (Miatanet), that should allow you to contact the relevant people.
You also probably know that there are at least a couple of LFX projects on MTnet, you could trawl that site for any insights into whether/how they handled ABS.
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