Does anyone know a good way to hook just after get_header() without hacking the theme? I know you can hook before the function loads header.php but I am trying to insert some HTML just after get_header() is finished.
I hate answering questions in the negative because it is easy to go wrong, but there is no core hook that does that-- at least no hook that I have ever seen. I admit it could be handy for a couple of things though.
It is possible that your theme has provided a hook though. I am thinking that that is the only chance you have of this working.