Perhaps I've misunderstood something - but it seems fairly simple - just use a variable (
$header). From TT manual (Directives):
[cut]
The INCLUDE directive automatically quotes the filename specified, as
+per INSERT described above. When a variable contains the name of the
+template for the INCLUDE directive, it should be explicitly prefixed
+by '$' or double-quoted
[% myheader = 'my/misc/header' %]
[% INCLUDE myheader %] # 'myheader'
[% INCLUDE $myheader %] # 'my/misc/header'
[% INCLUDE "$myheader" %] # 'my/misc/header'