So I found a glitch for sheathing unsheathable weapons. I was on the duel server when I screwed up and spawned with a pike instead of my lance and at first I thought that I didn't spawn with it. When i then get off the horse i scroll my mouse and i then hold the pike.
I then tried to swithc to my other weapon and the pike was dropped to the ground.
I picked it up and the got on the horse. Pike gets sheathed and not dropped.
Also tried with other unsheathable weapons and got the same result.
So basicly getting on a horse makes you sheath otherwise unseathed weapons. You can then dehorse and go around with them "hidden" in your pants or whatever.