This one was eventually an easy one to fix. Remove the resteasy-jackson provider and write a custom class that you annotate with the spring @Component annotation. Jax-RS picks up the provider and uses the object mapper in the provider. I was searching in the implementation, but the answer was in the jax-rs api.
@Component
@Provider
@Consumes({"application/*+json", "text/json"})
@Produces({"application/*+json", "text/json"})
public class JacksonProvider extends JacksonJsonProvider {
public JacksonProvider() {
setMapper(new CustomObjectMapper());
}
}