Testing the theme system: how?

http://drupal.org/node/333060 is the perfect example of something completely tweaky that is likely to get broken in the future when some well-meaning person goes through here trying to "optimize" the flow of things. AKA, something we really ought to have automated tests for.

In order to write a proper test, we would need the following:
a) The ability to make hidden themes like we have hidden modules.
b) The ability to enable those themes even though they're not in the UI.
c) The ability to test and display *.tpl.php files, parse template.php, .info files as sub-themes, etc. and basically do all the other crazy stuff you can do at the theme layer.

Anyone know if this is possible, or if not how far off we are and what it would take to make this possible? The entire theme registry is currently untested (apart from Views module :P) and there is some really complex logic in there.