sorry for the delayed response. I agree that if some SPI would allow to modify the builders, it should be done before the resource is registered in the resource factory. Do you think such a SPI would be useful for others?
I would be happy to contribute some code if it improves portability between RESTEasy and the MVC RI.
Thanks for pointing me to the resteasy-cdi integration. I'll have a deeper look at it in the next days.