I'm not a total expert but.... You can't actually make the back pack an item that you interact with. because if you did, the back pack would stay floating in mid air whilst the carriage moved on into the horizon. You could make the carriage an inventory slot of it's own though. You assign a slot and you make a pop up menu saying "ride or store you gear". If that's the sort of thing you want. But the original skyrim mechanics just don't allow you to do such a thing, unless you extend scripts and such.... but you would need to extend scripts quite a bit. Hope that you can do something with this, coz' as much as I'd like to make it for you I just don't find it a worthy project for me to embark on.