Just in time before the release...
It was a short night for me, but today's releases will provide full Universal wearable support in OpenSim v0.9.1.0, with improved bake-on-mesh support (proper refreshing of your avatar's attachments textures after rebake, proper support of Universal non-avatar bakes) and full compatibility with old viewers (automatic rebake when switching from a region to another and when they do not both support the additional bakes).
I tested against Singularity and Firestorm, with TP in/out of regions with/without Universal bakes support... Everything seems to be working OK.
The Cool VL Viewer is now officially the first viewer to fully support bake-on-mesh in OpenSim.
Thank you for your help !