Okay, so a quick look over the code basically turned up this:
Vehicles are essentially "owned' items. When they are first placed, the player who placed them is set as the owner, and a password hash is generated to prevent easy hacking. Only the player with the ID string has the correct password hash to interact with the vehicle.
However, it looks like vehicle ownership is changed when a player locks a vehicle, so here's what I would suggest:
1) Put the stuff from your inventory that you want to swap in to the vehicle inventory.
2) Unlock the vehicle.
3) Have the player who is going to be accessing the vehicle inventory lock the vehicle
This *should* make them the owner of the vehicle, and they should have access to the inv then. Just repeat the steps to change ownership again.
It's not great, but it's a functional workaround for the time being, if it works.
Do let me know if it does