That's a peculiar ruling, and might be something people object to. There is no Mymeara Craftworld list--it's not like Bel-Annath is part of a different army or variant list like the Corsairs are, he's just an Eldar unit. It'd be like a Space Marine player taking Calgar as an "ally" to his Space Marine army list. That said, there's already no reason you couldn't field Bel-Annath and Eldrad, they're single HQ choices in the same army.