So perhaps as a general solution:
1) add an acs-templating parameter to turn on the feature that will...
2) check for the existence of a package instance parameter that points to an optional adp location so search for adps. If a template exists in the optional location, that one will be served, otherwise, it uses the built in package template.
If there any opinion on whether this should go into the core, or maybe it could be an optional package that could be installed to add the feature to acs-templating.
That opens up the idea of having packages containing smaller pieces of functionality. I think that is probably a good idea to be discussed in another thread.
