This is an interesting idea. The dead-simple solution would be an event type that contains magnet links with a description. Trackers could post these automatically for newly seeded torrents, and/or someone could make a plugin for torrent clients that posts automatically when a client starts seeding.

I'd suggest magnet links over torrent files because torrent files with many files and/or trackers can be quite large compared to most Nostr posts.

This approach would cause active torrents to bubble to the surface. Nostr clients could save and index these events for later search if desired.

The biggest issue with torrents is verification, as you have no idea whether or not a particular magnet link contains what is on the label. This problem is inherent to torrents. As with URL links, it is primarily a problem of who you trust as your source.