I'm using a static body for the platform itself in that video - but the box is a dynamic body, and so is the player, and it works if the player stands on the box as well. I don't know how a kinematic body would change things.
I described it vaguely before, I didn't mean to imply that some central place kept track of all this. The entities (More accurately, their physics component) is responsible for handling the attaching, and for updating attached entities. I haven't yet made
entities detect their own motion, and move attachments in response to more than just the scripted motion. Currently strange things can happen when things on moving platforms collide with static world geometry in certain ways.
For a kinematic body, would friction cause a kinematic platform to move objects resting on it with it? If it did, that would be great. But it unfortunately wouldn't solve the problem of a dynamic body sitting on a dynamic body sitting on a kinematic platform.
I considered using a joint for it, but I'm not all that sure what joint to use, or how to detect when I need to turn it off. Does Box2D have breakable joints? A zero-axis-of-freedom joint that's easily broken might do it.
Ideally something like this would be part of the engine. (Well beyond the scope of a port) I don't intend to worry about the effects of differing friction, and how a low friction and a high velocity platform can break the attachment. But if it was done by
the physics simulation itself, it could handle all those details.