Instead of
tying in at the MFM signal level, why not replace the drive
controller chip with something that looks like it at the register level?
Because in vintage hardware, there's a connector for the drive, not
the controller chip.
Well, I suppose in devices that do use a (mostly) single-chip controller
in a DIP package, you could desodler that IC and fit a socket. That would
carry the necassary data, register select, DMA, etc lines. You could then
design a device that connected ot that socket in place of the original chip.
But it's not general-purpose.
-tony