Has anyone thought of or have an idea what it might look like if you would put the ability on a weapon instead of as a wonderous magic item? It would be helpful if you wanted to carry two weapons.
In general, this would cost twice normal. That's the usual modifier for slotless items and it's used widely for things like shadow and slick armor.
Tattoo weapon is an alternative that is slotless and doesn't cost more.
The Cloak of the sneaky scoundrel can hide a light blade.
The Scabbard of many blades isn't hidden, but can keep up to eight weapons handy.
You can wear up to two Vanishing sheaths
So with all of these options, you could have thirteen weapons available to you.