doing so would require a load of extra supporting circuitry, asynchronous bus interface, memory controller &c, all of which would have to be bypassed if you put an accelerator in.. it would increase the cost by about as much as an equivalent accelerator card, and would take up valuable space that could be used for something else. so i totally understand him not doing this purely from an engineering/design point of view. The only thing it makes sense to put on board from my point of view is an (EC?)020 socket, so it can function as a stock A1200.
there is already space for an FPU on a standard A1200 board, but nobody uses it because accelerator cards can't disable it if they provide their own. i don't know if this is fixable or not.