large foreign objects in LWW/LWL

Does anyone know if large foreign objects are well-supported by
LWW/LWL?  Specifically I'm thinking of arrays whose dimensions are
significantly larger than a fixnum.  If these are meant to work (they
seem to do so empirically) does anyone know if there's an efficient
way of iterating over them short of writing everything in C.  I
thought of having C code which drove the iteration then called back to
Lisp passing a pointer, but that might cons more than the naive way (I
haven't tried though).

I guess I should add the inevitable: LW *really needs* bigger fixnum /
array dimension limits on Windows and Linux.

--tim