First thing I'd try is riding it with the screw removed, bumpier the road the better.
If that doesn't work, then with extreme caution and at your own risk:
Thread the screw in from the other side. Put a coin, or some other flat metal object, between the two lugs. Once it's tight enough to hold the coin, an extra half a turn. Wrap some tape around the slot so it's sealed. Spray With Plus Gas (Other penetrating oils are available, but they're not all equal), leave for 24 hours. Tap it down with a rubber mallet, don't whack it, twenty or so taps, you're only trying to break the grip, a few taps at 45 degrees won't hurt.. If it moves, add more Plus Gas, wait, and try pulling it out, twisting, don't force it. more plus gas if it's moved but not free. If it doesn't move, another half a turn of the screw and repeat. Don't overdo the screw, if there's a gap between tube and seatpost, there is no advantage increasing it. Plus Gas works, you just have to get it where needed and give it time.
