Just discovered this FEP, it’s awesome, thank you so much for creating it @arnelson! Love how simple and clear it’s ended up, using well established attributes like rel for canonical and alternate and not needing any new properties. I should definitely adopt this in https://fed.brid.gy/ . It currently bridges web (via mf2 and webmention) and AP, I’m actively working on adding more protocols like you describe. Webmention isn’t in your list of example protocols, but it seems like it will fit fine...