Nope, it is not that trivial...
I tried it locally, but there is a problem with the parse directive, trying to embed unavailable files within the different themes.
#parse does use the $
but does not know what to use, if the requested file is not available.
If we want to address this problem these might be pssoble solutions:
o every theme has to include all template files so that the above parameter works as expected.
If a template is inherited, just use it as a wrapper for a parse call to the correct ('super') theme template file.
o rewrite or implement an "intelligent" parse macro, that takes the $parameters.templateDir as a default and tries to lookup
the theme inheritence hierarchy until the named file is found
What do you think?