I've found solutions for this, either by recording an stand anchor relative to the platform and setting the position of the player to it on every step, or using a PinJoint (tried with RopeJoint but didn't go well).
The problem is I need the player to collide with bodies welded on top of the platform, and:
- In the first case (setPosition) the player just ignores the bodies.
- In the second (PinJoint) the player pushes the welded body, causing the platform to move as well.
Illustration: (brown = platform, green = player, gray = welded bodies)

Is there any solution? Is there any way to make that forces applied to welded bodies don't affect the platform?
Thanks in advance.
PS: One solution would be to use a PinJoint for every body I want to be on top of the platform, so that their interactions don't affect the platform, but I think that would reduce performance (I guess a WeldJoint is more performant than using a PinJoint and setting it's target at every step, isn't it?)