Dear Lars Madsen,
> I'm not looking for a solution to the problem just an explanation as to
> how exactly mktexpk is called from pdflatex on Windows.
I don't know texstudio and your problem, but mktexpk
is called like
retvalue = _spawnvp(_P_NOWAIT, $SELFAUTOLOC\mktexpk.exe,
(const char * const *) args);
Best,
Akira