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Yeah, in the downwards direction onto the seatpost. The saddlebag would be pulling the rear parts of the rails, which aren't designed to be clamped (they are a different shape). The more I think about it the more it should be ok. Ultimately the saddle has a limit of 100kg and all I want to do is hang another 5kg or less off it. I'll just wrap some tape over the rails to prevent any rubbing.
Philip_J_Fry-
The forces you are putting through the rails will be all over the place if you're using the bike offroad so the rails should be designed to take forces in more than just straignt down like when you're riding in the saddle on smooth tarmac. Like you say an extra 5kg isn't much, and if the bag is fixed to the seat post then some of the forces will be going straight onto that.
Arducius
The saddle takes your full bodyweight on the rails, no?