In a somewhat similar vein, replace_header() retains the original header order.  Not quite what the OP wants, but useful.

The problem I had originally with a position-aware method is getting the API right.  I didn't want to add a position argument to add_header() (and still don't).  So I think a position-aware API would have to be a different method, perhaps with an API somewhat like list.insert()?  It doesn't look like OrderedDict provides any guidance here.