It is available, but only after you write the new stack poiner value to sp.
example:
; allocate 256 bytes from stack
lea (-256,sp),a3
; can't poke the memory at a3 yet, since any task switch
; would write registers & system context to stack overwriting
; our data!
move.l a3,sp
; now the memory at a3 ... a3+256 can be used freely. Any
; context switch will use memory before our data, and thus
; it won't be erased.
;....
; pop our temp storage off stack
lea (256,sp),sp
rts
If you're running low on standard 4k stack, just AllocMem/AllocVec some storage for the variables.
