The most complete up-to-date branch for the next Flambda is the cps_types branch here:
This is still very much a work in progress mainly by Pierre Chambart, myself and Leo White. We also have on board a couple of new developers who are being funded to work on the project: Xavier Clerc at Jane Street and Vincent Laviron at OCamlPro.
There will be efforts made going forward, from later in the summer when people are back from vacation, to be more proactive about providing information to the community as to the planned features.
The amount of design work required and current state of the project mean that we're not really in a place where casual hacking will be productive. Once things settle down that will change. However, in the meantime, if someone suitably qualified is capable of devoting substantial time to the project, they should please get in touch with either myself or Pierre Chambart.
Everyone's patience with this project, which is not going to make OCaml 4.06, is much appreciated. Early indications are that it will provide a step change in the optimization capabilities of the compiler.