when you are about to enter battle, your inventory is perfectly accessible. Problem is, that with a melee weapon, a crossbow with 20/40 bolts isnt going to achieve much. 60 bolts no melee weapon, only to pick up from the ground ? But you can have this an an excess of 96 arrows btw.
For siege, the inventory is too small, when you really want to use crossbow. There is no inventory to access as in field battles in warband so far. This could be a thing, so if you dont want to use siege equipment, just skirmish to soften defenders, if assaulting is actually difficult. Or have arrow/bolt buckets to get to the mantlets orr something.
then you might have some weaponry that is better suited to sieges like great crossbows, if they make it in the game that is.