Don't know what would work with the Manfrotto System, but I second the vote for the "L-bracket" from Really Right Stuff. I have RRS L-brackets on all my camera bodies.
I think several brackets for the Arca-style mounts offer that L-bracket type capability (RRS uses an arca-style mount). Downside is you have to change out your entire quick release system. Upside is it is rock solid compared to a standard plate system.