as mentioned on hipchat, an idea could be to rely on the ResteasyProviderFactory (which has a getInstance static method) and install a dynamic client feature in it (e.g. using registerProviderInstance(Object obj) method).
Perhaps you can try that and see if it fits your scenario.
For the executor, I'm afraid I don't see an already existing solution, besides having your own ClientBuilder that overrides the executorService setup; perhaps we can make this configurable in the ResteasyProviderFactory, though.
If someone from the team has further idea, just reply ;-)