Using a gang scheduler for SMP would allow Forbid/Permit-style disabling of the multitasking to be implemented. Nasty code that uses Forbid to speed things up will have the dubious distinction of being slower though because it will still run on one core only.
Personally, I think the FPGA approach to expanding the Amiga (eg. making slave processors to the master single-threaded CPU) is the best way but SMP is certainly possible.
There's no reason why AmigaOS couldn't do something entirely different. More than a few folks are working on system designs that don't rely on cache coherency and related paradigms to do local parallel processing. And I'm not talking about co-processors or other types of dedicated out-of-band hardware.