This can be done using the existing system, although it would be easier using this change.
1: Use hacky methods to add a element to the Avatar GUI.
2: Transmit it to the server in some way, either with normal avatar transmission (appending the new element), or making your own command.
3: The server mounting the object to the player, possibly offsetting it to be located properly if needed.
Although this will only work on servers with the add-on, and probably will have some bugs, this engine change is not needed, only for 'easyness'.