wow... love your bike but am sorry to hear about all your seatpost problems. and now you say it is down even with the clamp. at this point, if i were you, id see if i could get some kind of blade (slim flat head screwdriver, perhaps) to see if i could get something between the post and where it slides down in the seat tube to make a gap, but without driving the post down farther, and get more penetrating oil between the two to really eat at the rust. an easy out would be ideal, but not sure if they exist in this application. a fat enuff drill bit to drill it out, but you'd risk damage to the frames seat tube if your wernt spot on on your drilling.
something similar happened to me where the seat post was stuck in like yours. on the back of the seattube is that gap cut out at the factory to allow the seat tube to tighten up around the seatpost once in the frame. follow me? anyways, i took a hacksaw blade and alot of patience to cut a gap in the back of the seatpost, following the gap i mentioned on the tightener clamp. since the frame wasnt but an 80's murray, i even brought the gap line cut a few millies lower on the frame to get more of the post cut, and in doing so either allow less friction in the seattube and/or allowing more (i used liquid wrench, but everyone has a fav) pen oil to get between the clamp and seatpost. i even used a punch to "curl back" parts of the seat post by the cut i just made which also reduced contact it the clamp. i let it sit and the next day ui worked it out slowly.
good luck.
