afaik afma has nothing to do with hardware, but is a simple software hack that remembers the data per lens and adds/substracts this value from the values it's getting from the phase af.
I wouldn't assume that at all. My guess would be that autofocus is running on its own ASIC, and the code on said chip needs to know about the fact that it's supposed to accept offsets. If the chip wasn't designed with that type of input in mind, it may well be that it only has pins capable of starting and stopping the focus system.
At least, that'd be the cheapest way to build such a thing. Why add hooks for knobs that you know will never get twisted?