There are ways to replace the 64K RAM with 256K, but most software does not use it. The 1984 Byte magazine article requires machines with 74LS158 multiplexers for the DRAM address.

It piggy-backs on the DRAM address multiplexer U27.

There are two circuits, depending on the type of U7. 

Figure 1 for the CO21697 

Figure 2 for the CO12296

My machine has the CO21698 and the FREDDIE chip which replaces the DRAM address multiplexers , thus the Byte magazine circuit is not compatible. I would need to redesign the circuit. I don't think it would be very hard to do.

I might piggy-back a 512K RAM chip over the OS ROM chip. Twice as much RAM, plus the original 64K RAM.