I did this in the past, it was easier with cartridge BBs, goto the hardware store and buy a length of steel cable. Many seatposts have a hole near the top where the clamp is, if not drill a hole large enough for the cable to pass through, loop one end of the wire around the seat rail and secure with a crimp on holder. pass the cable through the seatpost and down the seat tube and out through the bottom bracket (you need to remove the BB). Set your seat height, make a second loop at the other end of the cable around your BB or in the case of an outboard bearing crank, make a loop large enough to allow the shaft of the crank to pass through. Reinstall your BB or crank with the cable looped around the BB or spindle. You can leave your quick release in place to allow you to lower your saddle, but you cannot remove the seatpost/seat without taking the BB out first.