Modifying Mastodon to allow 1024 character toots rather than "just" 500 (still way better than 140/280 cough cough) was fun, diff here:

https://gist.github.com/lindgrenj6/d2846a135656cfae20960b2fa156d62c

After the diff is applied, a `rake assets:precompile` recompiles the javascript and the frontend should _magically_ accept whatever you configured as the new toot length!