Weird ideas burbling in my head. Maybe a little go server that can be multiuser (but few users) per instance.
Post via bookmarklet, browser extension, or DM from another fediverse account. Maybe login via a DM'd magic link?
Emit activitypub posts for new links posted, ingest activitypub posts from followed actors for federated link sharing.
A SQLite DB per user with one shared DB for aggregate views. Want your data? Here's your DB file for download.
Maybe try a federated search thing where a query is gently requested against the instances of folks you follow.