Sébastien:
I'm chiming in late, but doesn't AIX have something like LD_PRELOAD?
Why not use it to transparently replace AIX's legacy malloc by another malloc implementation like dlmalloc or ptmalloc?
That would not require any patching of Python, and could also be used for other applications.

As a side note, while mmap has some advantages, it is way slower than brk (because pages must be zero-filled, and since mmap/munmap is called at every malloc/free call, this zero-filling is done every time contrarily to brk pools). See http://sources.redhat.com/ml/libc-alpha/2006-03/msg00033.html