Sounds like a bad idea, but what I'd say is create an object that would be the equipped weapon. Then when the player is equipped, you can put them into the equiper's contents, so they'll see themselves being carried around.

I actually find this idea usable, even though some find it a bad idea.

It's design concept could be as following:

The player develops their ability to turn into a weapon, customizing it's looks and it's abilities and learning powers, which they can use in their weapon form. Thus becoming an intelligent weapon with the ability to buff, empower their wielder or even use various attacks. The weapon could choose a master based on their own free will, when someone picks them up, whom you don't want you just "reject" them and they will auto-drop it.

When the player has turned into a weapon they create a weapon by the description and powers they had made, they themselves become a spirit of sorts, a ghost if you may, which can't leave the vicinity of the weapon. The weapon could be picked up and used by another player, while there is a link between the spirit form player and the weapon, you can choose to lend the wielder power or not, etc up to the player. The weapon would be dropped when the wielder logs out or destroyed when the weapon character turns back into human form. The spirit form is invisible to the naked eye, while only visible to the wielder, there can be telepathic communication between those two.

Rather than a straight up rejection, it might be interesting to allow the weapon/player to try to influence the wielder in a negative manner. 'Accidently' miss badly and injure the wielder, weapon powers backfire, taunting opponents giving them fury bonuses, etc. Annoy the wielder enough that they drop the weapon voluntarily.