The Commodore 128 hardware allowed two things that weren't possible on the C64: Moving the 6502 zero page and 6502 stack to another place in RAM besides pages 0 and 1, and switching the CPU and/or the VIC-IIe to access a second block of VIC-II color RAM. As far as I know, moving the zero page would not affect the fact that addresses $0000 and $0001 were mapped to the CPU's built-in I/O port rather than RAM.
Do you know of any software that actually made use of those features? Second question, were they (or either of them) used by the built-in firmware or the included CP/M system in any way?