This is just an experimental idea at the moment. What if plugins could register custom template loaders such that this worked?

cat file.txt | llm -t gh:simonw/summarize

Where the gh: is a prefix registered by a template loader plugin, and simonw/summarize is the string passed to that loader.

In this case the GitHub one could look for the summarize.yaml file in a simonw/llm-templates repository.