well i dunno how i did this but i had the silver shafted arrows from valentines day in my bank and when they were suposed to go away my account got frozen from not paying. 5 months later i unfroze it and the arrows are now permanently on my accunt even though it still says duration 7 days... i am now selling the flying valentine pet goblin things for about 100g each so imagine what would happen if u did this with brooms, im gonna keep mine forever.
method 2: "i havent tested this yet but im assuming it most likely will work"
wrap your broom in a wrapping thingy and then it willl be there when u unwrap it 5 months later
method one i know for a fact works, i havent tested method 2, have fun