How to lower a tandem bike seat past its limit?

My boyfriend has an old tandem bike (over 20 yrs old) which we have been riding - however I am short and my seat, even though its at the lowest possible position, is just a tad too low. the bike is an older so we can't find cranks which might have helped. also the shoes lock in so I can't really glue a block of wood on the pedals either. Any ideas?