I am unsure what this means. Firstly I think this functionality is way overdue. Secondly I see little value to having anything that is a pseudo-standard. I'd rather see the effort pursue another path to standardization or remain completely independent while clearly pointing out what the obstacles to standardization are/were.
On the other hand if this is required to be implemented in Java EE 8 runtimes I don't think it matters what the particular structural gymnastics are. Purely from a JCP standpoint, it is possible to have a sub-specification but only if the eventual goal is to spin it out into a separate specification. Examples of this are JPA, interceptors and others. There is no precedent for an "incubator" specification that is somehow optional. In fact I am pretty sure the current JCP rules would disallow this.
At apache we have subprojects which can become top level project if they behaves well. Would it be possible at EE level? Idea would be to have cdi-concurrency subspec (under CDI umbrella but not part of CDI itself - know there was appendix in bval for instance). Then for EE 9 if the subspec is proven as good it would become its own spec. wdyt?