I'm unable to recall a situation where the King or heir to a Kingdom was made the Byzantine heir historically, besides Bela III.
I'm very curious, what if Manuel has no sons and Bela-Alexios retains the position as heir - would the Romans accept him? If they did, what would the long term impacts on the Empire for essentially inheriting Hungary, or Hungary inheriting the Empire? Would Bela unite the administrations? What would be the result of an Arpad dynasty?
(The obvious benefit, IMO - is that it makes an alt-1204 unlikely if the two realms stay together, Venice would be right on the Byzanto-Hungarian border.)