I should probably have both what's there and include the relevant bit in MergeConfig as well?

To answer your question: images currently don't have lxcTemplateText, but adding the relevant bit in MergeConfig would make it "just work" when they do.  Not sure how it's template would get set though unless a new Dockerfile command was made.  Does that make sense? (I'm still fairly new to the docker codebase and not 100% sure I've got the full mental map of it).