> As it seems, it is not possible to run OCaml-code linked with thread
> support while letting C-threads perform callbacks.  This has already
> been a topic on the list a while ago.

I don't understand what you are refering to. I don't think there's a 
problem as long as you take care to acquire ocaml's global lock to 
exectue the callbacks.

#include <caml/signals.h>
...
caml_leave_blocking_section();
callback(*caml_named_value("myfun"), Val_unit);
caml_enter_blocking_section();
...

This ensures that callbacks into caml are properly serialized.

Daniel