So you could lift those 3 pin inside the dreamcast and connect both chip /CE line and the D501 point to those pins. Then wire the switch like normaly but inside you sd cart adapter. But this assume that your serial cable have pin and cable for the unused pin of the serial connector, which is unlikely I guess.
The problem is that the dc will not work correcly without the SD card adapter (Both chip always disabled because of the two pullup resistor), I guess the DC need to switch off the bios, using D501 link, at some point so another chip can use the bus. (otherwise /OE would be always connected to GND.)
So in conclusion that's a bad idea, but like I said last time once you flashed the bios you don't need the original bios, so just leave the switch inside the dreamcast.