Yeah, eladrin are going to get shoe-horned in. It's just a question of where.
The deathless' connection is with Irian (? the positive energy plane), not Mabar - the shadow/undead-lovers were killed off or chased off the island (House Vol & their followers). Which is part of why Eladrin aren't such a good fit for the "prime" elven cultures, as the Eladrin are explicitly tied to the fey realm (Thelannis in Eberron) and the Eberron elves have other connections.
Eladrin as not-so-common visitors from Thelannis would work, of course. Kinda like the Dragonborn suddenly showing up from wherever they're supposed to be living in the world (Argonessen?).