Simple,don't hang so much weight on the post in your backpack to begin with. Get used to packing light, as they say "take half as much stuff and twice as much money." You would be surprised how much you don't need to take with you.My Brompton in shopping cart mode can handle a shopping bag or my largest pack hanging on the seat back with 20+lb.I leave the handle bars up as a counter balance and it will stand alone with out any help.If you tend to pack a bit more than I do then put the heavy stuff up front on the luggage frame.
Besides don't forget once you get into the saddle for a days ride how much is weight is that seat post dealing with then?

Edit: I forgot to mention if you are worried about leaving a heavy pack hanging for some time on the back of the seat post than just lower the seat post until the pack is resting on the ground.