Or toss some TNT through...Build some Iron Golems on the Overworld side and push them through the portal?
Vanilla portals can act like chunkloaders, if you toss an entity through periodcally. I'd have probably set up a dropper with a 30-second timer aimed at my Overworld-side portal and loaded it with a couple stacks of cobblestone, and just let it run. Hopefully, during that time, the mobs would all despawn or return to their blocks.
Failing that, my next attempt would be to set up another portal a ways away (far enough that it didn't connect to the original nether-side portal) and work my way through the nether back toward the original portal, either killing the mobs camping the original portal or just drawing them away.