No direct experience of the D600, but I have used both RRS and Kirk L-brackets with various bodies (D200, D300 and D700). Both brands seem to have taken precautions for use of the connections on the left-hand side of the camera, although the rubber cover is difficult to access/open/close with the L-bracket on.
And I have never had to renounce my LCD cover. But once again, it has been harder to remove it with the L-bracket on, for cleaning for example.
If I may ask, why do you want to access the USB plug? If it is for shooting tethered in a studio, I agree you don't really have a choice, but if it's for downloading purposes onto the computer, I dare say a card reader is a much better option, the transfer is much quicker that way!
Hope this helps,
Olivier Rychner __________________________________________ Jetez un oeil à ma galerie if you feel like it! And it's a bit void as of now, but I also have a Nikonians blog
Auta i lomë! And my Nikon's only awaiting daylight...