The L2 Slot carries enough signal and addressing lines that the entire computer can be run from it as in the case of the Crescendo 1 MB L2 G3/466 in my 6360. IIRC, the PCI architecture version of "Designing Cards and Drivers for the Macintosh Family Hardware" (?) is available in .PDF format on a Developer CD. All my books are still packed in boxes from my recent move, so I can't check ATM. Designing a new CACHE DIMM would probably be easier than converting a ROM DIMM.

OOPS! Shoulda' mentioned that it's a DIMM module up front! That's why the signals on the two sides are independent of the opposite side.