This is a fun one. First user with an underscore in their username signed up just now, https://me.dm/@punkscience_ns , @punkscience. (Hi Darryl! Sorry for the trouble!)

We translated the fediverse address @[email protected] to the bridged Bluesky handle punkscience_ns.me.dm.ap.brid.gy and created the DNS TXT record _atproto.punkscience_ns.me.dm.ap.brid.gy, which is serving fine. PLC directory has it in https://web.plc.directory/did/did:plc:di27vbfe6wutukod4g36kovz.

However, ATProto handles don't allow the underscore character. 🤦 (Also see swicg/activitypub-webfinger#9) So, I guess we need to translate underscores to maybe dashes, and hope we don't hit a collision between eg x_y and x-y on the same instance.