If I transfer a Groudon from a Pokemon Emerald into my Pokemon Platinum, and then use Pokemon Bank to transfer it to my Pokemon Alpha Sapphire, would that Groudon be able to go through Primal Reversion?
Yes.
You can transfer any Pokemon from previous Gens (uptill GBA) to XY/ORAS and they will still be able to Mega Evolve or perform a Primal Reversion.
