Thanks. The moment I posted, the idea of adding the platform behaviour came to me. It works nicely now. Changing the gravity angle also creates a wall trap, so I'm happy with this solution :)
EDIT: Another problem raised up. When using the platform behaviour on several instances of an object, there's a problem I can't solve:
"Platform on landed - then the code", BUT, it's not a per-instance condition. It only considers the landing ok when every instance of the object is touching the ground. If I use the "For Each" loop, it tells me that a trigger can't be a subevent in a loop and it kinda crashes.
It seems like "platform on landed" doesn't work the same way as "on collision". Problem is, as I said originally, collisions are not working well in this particular case.