And why is the split 12v rails a requirement? Why does the sweep supply have to be separate? They dont have to be separate if you put separate EMI Pi-filters on the output for each circuit path direction, and use diode clamps for voltage transients, so noise dont get back into the mobo from sweep, and vice versa.

Steal ideas from the automotive world, There is only 1 supply, So look at the different methods used to eliminate noise in that regards.