68020 and greater Macs need a hardware widget to remap their RAM into a contiguous block (be that the special stripped-down remapping chip in the Mac II, the version of that built into the LC, or the onboard MMU in the '030 and later systems.) and the memory maps for the two modes are substantially different. (I/O gets shoved to the top of the 32 bit address space, etc.) This is why even "real" Mac II family systems can't use more than 8MB of RAM in 24 bit addressing mode, and why accelerators for 16 bit Macs that include onboard RAM sockets are generally limited to using anything above the 4MB mark for RAMdisks, etc.
I do recall reading about some software for Mac II's with full MMUs that would let you get up to 14MB of RAM in System 6 by doing some really, really horrible things; IE, copying the MacOS ROM up to RAM destined to be mapped at the 15MB mark, patching it so it worked there, and using the MMU to map RAM on top of the 24 bit address spaces for the NuBus slots. (I believe to use this software you actually had to rearrange your NuBus cards so they counted down from the top slot, and each card you used knocked 1MB off the maximum RAM you could have.) In principle I suppose you could do something akin to that and relocate the Portable's ROMs upward, but you'll still need an MMU.
(Edit: Someone undoubtedly knows what this software is; again, I read about it *ages* ago and am undoubtedly misremembering details.)