What would be interesting is body part Add-Ons with some sort of structure like this:
Add-Ons/BodyParts_Default.zip
description.txt
server.cs <-- This file is empty like decal add-ons
/LHAND
LHand.dts
LHand.png <-- The icon that shows in the Avatar menu
LHook.dts
LHook.png
/LLeg
LShoe.dts
LHand.png
LHook.dts
LHook.png
(New body parts come as an Add-On organised into folders for the node names, then your body parts are selectable through the avatar menu. Not-found ones in servers are replaced with the default appearance.)
I doubt it'd be possible (efficient) without lots of engine changes, though.