There's really not much extra complexity: instead of usernames which are of the form tel:+31123456789, one could also support usernames of the form mailto:foo@e

There's really not much extra complexity: instead of usernames which are of the form tel:+31123456789, one could also support usernames of the form mailto:foo@example.com, urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6, along which canonicalisers & verifiers for each (the last might be some sort of internal Signal-scoped hash — I dunno). Yes, it is some, but it's not a ton.