You can disable the physic behavior for all objects with Y less than your character. Problem is, that other objects ignore those to, so you have to add an invisible sprite/TBG with physic behavior, lay it over all JumpThru platforms and disable collision for your character. The reason why you can not deactivate the collision of the JumpThru Platform for the player right away is that this action always counts for all instances and is not applicable individually.
+ System: On start of layout
-> PhysicBlockInvisible: Disable Physics collisions with Player
+ System: For each PhysicJumpThru
----+ PhysicBlockJumpThru: Y < Player.Y
-----> PhysicBlockJumpThru: Set Physics disabled
----+ System: Else
-----> PhysicBlockJumpThru: Set Physics enabled