Theres a lot more than {GEEKLOG_HEADER} in overall_header.tpl. {GEEKLOG_HEADER} is simply the Geeklog header and the left blocks (if you chose to include them). If you want to put something somewhere inside that header, then you will either have to put it in your Geeklog header (the Geeklog header.thtml file) or you will have to remove the {GEEKLOG_HEADER} variable and create your own phpBB header without the Geeklog left blocks.
If you put it in the Geeklog header.thtml file, the new code will appear on all of your headers, not just the ones on phpBB pages. Is this what you want?
