You can trade national Pokedex Pokemon to Platinum or Black so that is alright. The only game where this isn't the case is in Fire Red and Leaf Green.
I don't think it's possible for either game to trade over a Pokemon that isn't in its regional Dex until the national Dex is obtained. You certainly can't transfer up until then as the Pal Park/PokeTransfer facility isn't available until then. Part of X/Y's gimmick is that in part because of Wonder Trade it is possible to receive Pokemon that either wouldn't be available until much later at a point you haven't reached yet, or not at all such as the Speed Boost Torchic event that ran around the time of the games' launch. I would suspect this will hold true for ORAS as well should they retain Wonder Trade.
...Or maybe it is. Durant probably knows better than I do about this.
Nope, I can attest that Durant's completely right. You can trade any Pokémon to any game except in the cases of trading non-kanto Pokémon to RBGY or a non-national dex FRLG.
Thanks for the answers, i shall complete my next platinum run with a bellsprout :')