is something like this what you mean Steve?
I cobbled together a not too particularly efficient M/S bus routing, but one that allows you to track what is going on during the whole signal processing thing...
I've rigged it so that all you have to do is drop any stereo file on to the first track, leaving you to adjust the volumes of the separate MID and SIDE signals, the red channels, to see what is contributing what to the stereo signal.
If you go to screen set two, you will see what the mixer is doing to split up the stereo signals and what is being inverted... or what isn't!
As Steve pointed out you can also filter the center frequency before sending that off for M/S trickery as well to try and target only the vocal frequency range because you'll notice that in some cases, not a great deal is left in the side signal.
Incidentally, you might want to check out your pan laws found in the project's audio settings, not the preferences of Logic itself.
If all of that looks like too much hard work, maybe someone will make a macro out of it if that would help... or.... you could try this free plug in from Voxengo! They recently ported their M/S plug in to the Mac.... and did I say it was free!? It does M/S to Stereo and back again at the touch of a button with independent gain controls over the MID and SIDE signals.
Incidentally there are a few M/S plugins coming in to fashion for mastering too. Ozone got updated to version 4 recently, and that now has M/S processing built in.
Feel free to make suggestions to the project too.... as always there are many ways of doing things!