Do you mean something like this:

Template:

    blah blah [% user.name %] blah, blah.

Test data:

    $$vars{user}{name} = "<<<<<< user.name >>>>>>"

Template produces:

    blah blah <<<<<< user.name >>>>>> blah, blah.

Testing code:

    ok( $output =~ m/<<<<<< user.name >>>>>>/ );
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However this method could easily lead to very involved tests for complicated pages - and testing the test code could become trickier than testing the template.

Also if I have understood you correctly it will make it difficult to test real world data. As I see it the only really good way to test the templates is to create simplified ones and then to check the output against known good results - even if that is more labour intensive.