I think this is unnecessary, malloc (either the bash malloc in lib/malloc/malloc.c or the libc provided malloc) should already take care of requesting memory in page sized chunks.
At least that's what I see here (morecore function): http://git.savannah.gnu.org/cgit/bash.git/tree/lib/malloc/malloc.c?h=devel&id=216e2e9b8ba21fff677cf7794ef3d9af8c91d46d#n622 Unless I'm missing something, there's nothing to gain from optimizing on top of xmalloc/malloc.
