I got started on an, in hindsight perhaps overly ambitious, card that would have sockets for both FPU and ROM expansion. (The two expansions are more or less completely orthogonal, only sharing physical space.) I got a bit bogged down in trying to choose a PROM technology. Flash would be best, obviously, but is somewhat challenging because write access to the expansion slot is not supported, so fiddly schemes like reading from unlikely sequences of addresses that as a group encode the address and data to be stored are required. Worse, my clever plan to use traditional UV-erasable EPROMs and add complexity only after I got that working encountered challenges because very few of the usable sizes are still available enough to buy in quantity without having to pay shocking amounts per chip.
In short, I’m still working on it in theory, but due to time constraints it won’t happen soon.