There actually are a couple commercial solutions for replacing MFM/RLL hard drives with either SATA or flash... but they are turn-your-hair-white expensive, unfortunately. (North of $2000.)

Its something still just a little out of the "easy homebrew" ballpark, alas. It would probably be doable with an FPGA but it requires handling serial data at about 20mhz with no latency. (Or to put it another way, it's about ten times harder than a floppy emulator.)