> I don't yet see why would you need to send data to a module.  Why not
> generate that data in the module code itself, based on some Lisp you
> get from Emacs?

I don't see why you would need to question my approach: Not all data can
be known in advance and it would greatly diminish the utility of a
foreign function interface to limit itself to strictly dealing with
static data.

In any case, Stefan answered the question satisfactorily: It does make
sense to have an equivalent API to transfer unibyte data back to Lisp
code.