Ok, seats are at HMS and are on their way to mine. Attached are the pictures HMS sent me. Their recommendation was to just buy new heaters and put them below the pads, and I'm likely going to buy third party ones (the F87 seat heaters are like $150/side and a very odd shape that's unlikely to fit well in the shell) and replace the plug with the BMW plug.
I'm actually attempting to build my own brackets (laser cut, bent, and powder coated 1/8" steel, similar to the M4 GTS brackets), so that's going to take a little bit of time. I need to test fit it before I make the remaining ones.