@dalias @pwaring No, there isn't. But that is something that would require Mastodon to support, not something I can easily do on my end.
I don't develop Mastodon. I'm not even much of a code contributor because I know virtually nothing about Ruby/Rails.
Even if that were possible, I'm not sure how much it would impact the user experience. Imagine opening the app (or every time you scroll) and having to wait for media to be fetched remotely and cached locally on the fly.