Hi tomf84,
At the moment we don´t support compression for websockets and it is unlikely that we will see that in the OSS version of Mosquitto.
What we have done for other customers in such cases: They carried some of 50% of the development cost, to push implementation.
So far it is not scheduled, but if you are in dire need of a feature like this, contact us at inquiry@cedalo.com and we can have a look on how this may fit into our roadmap 
.
Cheers,
Tizian