A pinout may actually be necessary either way. It is possible that there is a connection that signals to the Mac that this module contains ROM. Removing the ROM chips would not affect such a signal.

It may also be possible to find a pinout for the ROM chips themselves and simply disconnect a pin that enables output, ex. /OE, /CE, etc. With this method, it may be necessary to tie the disconnected pins to a logic level, most likely high, but unofficially, disconnecting will probably work. This can prevent removing the entire chip which is extremely difficult without proper tools.

If you can't find a rom chip pinout, many roms use the same pinouts, so with a little experimenting you'll likely figure it out.