I was wondering if there was any way to access the IQDB using the E621 API? I believe it would be convenient for that to be implemented if it already isn't. If it is, could someone provide a general basic guide on how to use it?

With a URL: https://e621.net/iqdb_queries.json?url=
With a local file? Absolutely no idea.

Make an authenticated post request with the body containing a file parameter for local files.