[Keeper of Dol Guldur]

Everyone seems to forget how weak Morgoth became at the end of the 1st age. Tolkien explicitly stated that he was weaker than Sauron was at his peak. And Sauron's evil self was definitly spawned almost directly from Morgoth's influence or driving spirit. But no, direct inprisonment of Melkor's spirit is pretty crazy. Though the One Ring could have been a conduit for Melkor to lend his power to Sauron, like jumper cables. Melkor was trapped in the void, Sauron wasn't. If he still wanted things to go his way, he could have ordered this plan to Sauron ages before creation of the one ring, just in case. That maybe, is a little more plausable.