Thinking about this, there just doesn't seem to be a solution. Header sets HTTP header variables in the document that goes back to the user's browser, not the second server. POST variables would usually be provided by the user's browser to the second server, not by the first server. I just don't think that there's a way of doing this kind of redirection, but I may be wrong, it's not really my area.
Fascinating problem, would anybody like to comment?