I can expose ALL of Datasette's functionality on the command-line (without even running a web server) by adding --get and --post options to datasette serve.

datasette fixtures.db --get "/fixtures.json"

This would instantiate the Datasette ASGI app, run a fake request for /fixtures.json through it, dump the results out to standard output and quit.

A --post option could do the same for a POST request. Treating that as a stretch goal for the moment.