Hi Francois,
thanks for sharing. We ended up with a very similar concept for our iPad integration for our configurator apps. We decided for the websocket based WAMP message protocol (http://wamp-proto.org/) and used crossbar.io as an external message broker. You wrote that you might add an http and json based implementation to the UDP Message, you should consider this. The benefit is that there are several libraries for that protocol already. Especially the javascript and python libraries are very useful. If you want to connect to maya or 3dsmax, everything is already there. We integrated WAMP into Unreal using the WAMP C++ Autobahn reference library.
We already connected Unreal to our logic database this way and we can build dynamic user interfaces for the configuration selection with hundreds of configuration options per car from the database now.
Cheers,