For performance reasons, I need to locate task stacks in a small amount of internal 32-bit wide CPU RAM. The general heap is a large external 16-bit memory with wait states.

Is there a way to do this?

By selecting the appropriate heap_x filee for your project you can control how FreeRTOS gets its memory. heap_3 calls malloc/free getting from the general heap. heap_1 and heap_2 get them from a private array, which you hopefuly can force into this memory space with appropriate commands. This will move ALL FreeRTOS memory allocations into here (stacks, TCBs, and Queues). If you want to just move the stacks, you should be able to edit the create/destroy stack functions to call a different heap function then the rest of FreeRTOS, and the appropriate heap file to generate that alternate name to provide it.