Jochen: I'd rather you (manually) patch the dirmngr tarball included in
gpg4win-2 and create a testinstaller and try that one out.
I found the Problem in this issue and tested that the attached patch solves the
problem, yes It would have worked on GNU/Linux as the "b" has no effect there.
Finding out since when the problem existed appears moot to me and you would have
to check in dirmngr's SVN and likely always existed.
But maybe there are additional problems (as this is imo a very exotic feature)
so it would probably make sense to test it again on Windows before preparing the
next stable Gpg4win release.