OK so... I placed a temporary sprite at the beginning of the ramp that, if the player holds down the UP Key while overlapping it, it will make the platform solid and, therefore, make the player go up the hill.
If the player comes down from the hill and collides with this sprite, it disables the solid state of the said platform and the player can continue down the lower path.
These changes can be noticed with a change of color on the temporary sprite.
This seems to work perfectly at a singular level. The problem now is finding out a way to turn this into something global, avoiding using 4 events for every climb.
Here's a link for the capx: