Use of system variables

I had a go at using esxDOS with a ROM that stores the system variables in a different location. Unsurprisingly it didn't work. I wonder if it would be possible to change the code to always address whatever variables are being used via the IY register instead of at an explicit address. This would be useful for extending the amount of space available for BASIC in SE Basic IV. With the new screen modes it should be possible to put the system variables at the start of RAM and free up an extra 7K for BASIC programs.