From what I understand the L-plates and the camera specific anti-twist plates are only made for the Arca Swiss quick release system. Am I right? Are there no anti-twist plates or L-plates that are compatible with the Manfrotto system?
Manfrotto has L-plates: http://www.manfrotto.com/l-bracket-q2 (a few varieties).
Someone else has already mentioned the Manfrotto L-plates. Therefore a little more general advice:
Read this before you start investing significant money in camera support. Afterwards try to play around a bit with ReallyRightStuff, Kirk or similar quick release equipment. After that you probably don't want to touch most of your Manfrotto stuff again. The dovetail precision equipment is just so much better, functionally and stylistically.
Most high-quality camera support will outlive many camera body iterations (though probably not the camera specific L-plates), and tends to be a good long-term investment. If you sell the stuff again at some point it will still be worth quite a lot.
Personally I like the endless combination possibilities of ReallyRightStuff equipment, and its even inter-brand compatibility. As an example, here is a picture of an extra L-plate used to point the camera downwards without tilting the ballhead by 90°:
Bottom up this consists of tripod, ballhead, quick release plate, L-plate, two back-on-back mini quick release plates to connect to the camera L-plate, camera. Very sturdy, very adjustable, and a joy to set up... If I don't do improvised product shots like this, basically the same equipment serves me well for nature, landscape and panorama photos.