We have narrowed down the list of forked packages we use to just one, plone.app.blocks; our version handles tile rendering differently (faster, as I understand it), and we want to contribute this back into the master branch using @davisagli’s suggestion that we implement the changes via an adapter.
We have tested internally with Plone 5.1rc1 (and I see rc2 has just come out).
This is GPL2 so we hoped that by making it publicly available we might get contributions, and we thank those who have made them!
In many ways, CastleCMS is a working proof of concept for features that Plone has considered at one time or another. We continue to believe this will be the case going forward, even as we (as time permits, which admittedly is tight at the moment) contribute some of those features back into Plone. The code is there, and we invite others who have the time and the interest to take those features and put them into Plone, or to help us continue to drive CastleCMS forward, or both.