In reply to ice.solo:
have thought about this a lot myself.
I don't think you will have a problem with water between ledge and fly as the ledge should have drain holes in it.
I would have two concenrs:
1. not losing the tent in high winds when no one was in it, so you need to think about securing it to the ledge / wall.
2. the height that the ledge is positioned at in relation to the anchor varies the angle of the ledge suspension, which means depending on the situation, you have trouble squeexing the tent onto the ledge.
I think a full expedition fly would be better and leave the tent on the ground.
If you are going up and over, then the first light is less than 1.5 kg, so taking it in addition is not going to make too much difference.