As Microsoft documentation states (http://msdn.microsoft.com/en-us/library/cc817574.aspx) the META tags that direct IE8 must be positioned as follows: "The X-UA-compatible header is not case sensitive; however, it must appear in the Web page's header (the HEAD section) before all other elements, except for the TITLE element and other META elements.".
The most flexible solution would be to honor the order of META and other tags that author has put directly to the template: the place for T5 tags could be indicated with special tag:
For an example
<title>My Web Page</title>
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7" />
<link href="dirlang.html" rel="next"/>
If author decides to omit the t:headcontent tag then T5 would not attach any meta or link tags.
Other solution as Howard indicated in the mailing list would be to just add the T5 specific stuff to the end of the head section.