Hi Gobe,
I was actually looking for this function too. I was suprised this function wasn't there, I already made a custom model to attach to a bone. I tried to make a script to replace the function, but it isn't possible (well, with limited functions). It would be very nice if this function gets implemented into the next version. Anyway, the script I made got the bone position and the rotation of the player. Then it put the object at that position/rotation. It's very buggy, when you crouch the object doesn't move with the bone and when you walk forward you get the same problem. It doesn't refresh fast enough, but when he e.g. moves his head the object doesn't rotate too. So it isn't possible to imitate very well.
I also tried rigging my model to a ped, but then I saw that it wasn't possible to replace a ped model . So I think the only way is waiting...