I think that the only thing that prevents you from ignoring the memory controller and initializing the rest of x86 board while still remaining in the CAR mode i

I think that the only thing that prevents you from ignoring the memory controller and initializing the rest of x86 board while still remaining in the CAR mode is the sheer ridiculousness of doing that. As for whether you have an memory-mapped ROM available I'm not exactly sure, but the high-level model of what x86 firmware does seems to imply, that the hardware maps an part of SPI Flash at the address range where there was an ROM chip on the 8086/286/386 PCs (the actual address ranges are different).