Thanks for the reply Tom (and in reply to your message, I am modifying the interpreter)!

> The codesize is indeed reliable, though interestingly enough I get the
> codesize for _driver being 25 bytes rather than (0x24 = 36 bytes). I've
> just checked and 25 bytes is indeed correct (including 1 byte for the
> END_CODE itself). Here is a printout of the function, as loaded by the
> interpreter.

Aha, seems I threw something of a red herring with the 0x24, it was in fact decimal 24.
Judging from my current output (Warning, assembly): http://modseven.de/pastebin.php?id=1802
something went wrong around the NEED_HEAP_32 / UNPACK area.
I'll investigate it and see how it goes,
cheers,
  Stew.