They used to sell a little device... a "star-fangled" nut went into the seatpost, and you screwed an eyebolt into that. suspended from the eyebolt was a short length of cable and a plug that would fit into your seat-tube. Also included was an extra-long water-bottle cage bolt.
So, you'd drop the plug down into the seat tube, screw in the water-bottle cage bolt, and viola... The seatpost could be pulled up a bit, but would mysteriously stop.

I don't know if they market these any more, but it would be pretty easy to make one.