@brennen opt in through the profile directory would work. I was surprised that the four profile fields are free form. We could propose a standard “Twitter” key for it.
birdsite
@tannewt on mobile so not doing the requisite googling, but i'm guessing there's some prior discussion to be found and some relevant prior art in terms of the rel=me support for profile fields: https://docs.joinmastodon.org/user/profile/#fields
& yeah, if you work something up, i do strongly advise a consent mechanism not unknowingly triggered by any existing conventions. the culture has an intense aversion to types of algorithmic / scraped discovery mechanisms that are pretty standard elsewhere on the net.
@tannewt not offhand. something about the _idea_ of one kind of triggers my fediverse-would-yell-about-this-a-lot spidey sense, though a purely self-submitted opt-in one might avoid that, especially if it was attested somehow from user profiles or something...