Perhaps I've misunderstood something - but it seems fairly simple - just use a variable ($header). From TT manual (Directives):

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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'
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