These pages have draft sections from my thesis of possible relevance to JSR-286. Hope they are of use :) The final version of my thesis (PDF, 11MB) is now up too.
They describe limitations of JSR-168 encountered while developing portlets generally, and then when developing the IPC library. (Final summary at the end)
The IPC sections mostly concentrate on the design of the library, which will not be of direct use as it was constrained to fit within JSR-168, whereas the IPC in JSR-286 will of course be much freer to do what it likes!
However, our specific messaging requirements and use cases may be of interest.