Two questions. How much is this frame worth to you and what is the seat tube wall thickness.
Basic engineering - your post is trying to rock around the top of the seat lug when you hit a bump. The very end of the seatpost is doing most of the work to hold in in place. The (simplified) load at that end is what you are doing to your seat X the distance, top of seat lug to seat top / the distance, top of seat lug to bottom of seatpost. So, if you have twice as much seatpost buried, the force the end will see will be half. Why does this matter? Because that force is directly against the seat tube wall. Too high and the tube bulges. Probably not catastrophic and if your frame is built like a tank, probably will not happen. But if this is a sweet Columbus SP (say) frame, that bulge would be a deal breaker to a potential buyer and something you see every time you look at the bike.