For me your post is sort of self contridictory (as seems evident by the varied answers you've recieved) and as such needs clarification.
For example you mention the lack of a hole for the lock pin, but then say when rotated to landscape position. Unless I'm missing something, I would think the lock pin is more for portrait orientation than for landscape.
As others have indicated, the 70-300 (which I also have and use on my tri-pod) is not that heavy, but heavy enough. When I use it on my Manfrotto tripod and head, depending on how far from parallel it's set, I experience flex drop not rotation of head or plate. It's just a matter of getting used to how much drop for a given angle to allow for it when framing.
Other than the flex drop, I have never experienced plate rotation when using that combo in portrait orientation.
On the other hand with the two heavy lenses I have with lens feet for tripod mounting, they balance well with the D7000 on a tripod and I don't experience the same flex issue when framing.
So tell us a bit more about your setup. Your profile is blank so we have no idea what model or manufacture tripod and head you are discussing which may influence the answers you receive.