but i dont think there is away to control the speed of the EMB apart from system clock rate and the amount of wait states for access
Correct.

how ever i could ensure that certain code / process are run in the area
If you're talking about dynamically changing the clock rate I have a hard time seeing how that would work. I'm not sure how you'd locate certain functions in the external memory space, but beyond that you'd have to watch out for what happens during interrupts. ISR's (and all the peripherals) would run at two different speeds depending on when the intr occurred. You'd almost have to disable interrupts when executing code out of the slower region.