Faced with a strange address that goes beyond the Sega Saturn memory address space - 0xFFFFFF00. How to track read / write at this address? The Yabause debugger does not allow this. Maybe this address is a mirror of some other address?
Well, this is not an address, per se. It is a register. So the fact it isn't in Yabause debugger is their fault.
To be more helpful, this register has read and write permission. So you could create a hack which, at your desired point, issues the command
and then check what's in the general purpose register you moved it to.