Here's the link to the additional discussion about switching between 128K & 512K.
In addition I've been looking at H3NRY's MacDraw schematics which shows the 128K/512K hybrid board. It appears that if the W1 jumper, which terminates to ground, is not cut, then it bypasses the 512K multiplexer circuitry. At a minimum, it appears that a single DIP switch could be installed to switch the W1 between the ground and R42, effectively enabling or disabling the 512K expansion circuitry. But I don't know nearly enough about this to be sure.
In addition I've been looking at H3NRY's MacDraw schematics which shows the 128K/512K hybrid board. It appears that if the W1 jumper, which terminates to ground, is not cut, then it bypasses the 512K multiplexer circuitry. At a minimum, it appears that a single DIP switch could be installed to switch the W1 between the ground and R42, effectively enabling or disabling the 512K expansion circuitry. But I don't know nearly enough about this to be sure.