Great new addon appeared in 512 and thats avatar attachments.
This allows you to load a fbx model from a server and stick it to your body.
I made a low poly guitar a while back
Using the above settings I got it to attach
this wiki link has all the bone names if you want to attach something elsewhere https://github.com/highfidelity/hifi/wiki/Naming-Your-Skeletal-Joints.
Very cool new addon big thanks to who ever made it