I haven't tried this, but it seems like one way would be to make a module containing nothing but a single dummy variable. If you then arrange to have that module be the last one linked, a pointer to the dummy variable should point to the spot just beyond used memory.

There may also be a way to get the linker to define a symbol having the value you want. Never having messed with SDCC, however, I can't say for sure.

-- Russ