someone is working on a reddit api compatibility shim so reddit apps could connect to Lemmy without redesigning their apps
https://github.com/derivator/tafkars/tree/main/tafkars-lemmy
“Tafkars stands for “The API formerly known as…”, is written in Rust and is pronounced like “tough cars”. Tafkars is an API proxy that allows apps to talk to Lemmy through a familiar API from a kinder time. The hope is that this will make it easy for app developers to support Lemmy with only minimal code changes.” @fediverse
You could cheat and return subreddits like r/[email protected] but that’d probably break some clients that validate subreddit names. You could also try to encode stuff (i.e. r/fediverse__lemmy_ml, replacing __ with @ and _ with .) but then you’d quickly run into the length limit. Same for usernames.