If lemmy created a community the same name as the username when someone signs up. You could follow a lemmy user and then when your on mastodon (once we’re federated with mastodon) you could see their self posts in their community as a mastodon post and vice versa.
Federating with mastodon makes the most sense to me because its the most used and because the UI/UX would pair so nicely. A toot would be a post in your usernames community and a like would be an upvote. Replying to someones toot would be replying to their lemmy thread and likes on the reply would be upvotes. It just makes sense in my opinion and being able to view/comment in lemmy with a mastodon account would help grow lemmy and most mastodon users i come across are leftists and FLOSS advocates.