if the template does not contain explicit "templatedata", the API call should return auto-generated "templatedata" structure, distilled from the template itself.
it's trivial to do - basically parse the template, extract parameter names (including order based, aka 1,2,3), and return "default" templatedata where every param is of type string, without any special attributes (default, required, etc.)
this 15-line enhancement will make VE suddenly useful for templates, without detracting anything from current functionality.
we did this in hewiki with "template param wizard" (the equivalent of VE template editor, for the old wikitext editor), and it enhanced the wizard's functionality tenfold.
regretfully i can't submit a patch - hopefully someone else will be able to do so.