I'm not the "Gantry Guru" so please don't be offended, but you could make a simple file_exist call, if it is there you include it, on the end of the template-details.xml (maybe a feature Section) if not then template-details it is.
We'll certainly look into something along these lines. Probably for Gantry 5.0 as it would most likely require backward-compatibility-breaking changes.
i hope not, backward-compatibility is one of the major points to use the Gantry-Framework. Because of backward-compatibility we can use "older" (RT-)Templates on all Joomla versions.