Hi, and welcome to the forum billo.  I'm a little unsure on how it might not be legal to insert the code in to the header template?  I'd be interested to read more about that if you have a link.

I have used AdSense on phpBB v2 before and although it depends a little on the skin you are using, I was able to insert the code in to the file:

templates/<your_template>/index_body.tpl

at the very top of this file.  So I placed something like this in the very top of that file:

<table width="100%" cellspacing="1" cellpadding="4" border="0" align="center">
<tr>
        <td width=100% align=center>
        <script type="text/javascript"><!--
        google_ad_client = "pub-xxxxxxxxxxxx";
        google_ad_width = 728;
        google_ad_height = 90;
        google_ad_format = "728x90_as";
        google_ad_type = "text_image";
.
.
.
.
.
        //--></script>
        <script type="text/javascript"
          src="http://pagead2.googlesyndication.com/pagead/show_ads.js">
          </script>
        </td>

</tr>
</table>


That would give me a 728x90 ad block below the header of every page.

I always just throw the big banners into the footer and the header. Getting it into posts and such is much more difficult, because you have to edit multiple files. The header and footer are called on every page just the same.

Check out www.phpbbhacks.com for adsense mod! It worked on my site very well.

Most probably you were added to the xrumer database. No, I won't link... Check a search engine for auto submitter!

The best way to do this is to go to phpbb.com and go to their forums.
Check out the 2.x.x mods forum.
It's much safer to use an official advertisement management mod rather than edit the files directly without proper knowledge.