Apparently, this hard panned vocal and instrument practice was common on early (stereo) Beatles albums. I had access to a preamp for a while that had a unique balance control that allowed you to merge left into right selectively from a hard pan, to fully centered, and even to reverse. Really cool feature for those rare albums.
MC has the ability to apply DSP on a per song basis, but it comes with some serious setup and maintenance. You can mark a song to use any DSP present you want. BUT! As soon as you use one, the DSP will "stick" to that preset until it hits another song with a different preset, or you change it manually.
In practice what this means is, you need to mark EVERY song in your library with a base preset. Then apply special presets to songs that need them. This, for me, is way too much work, especially in an ongoing fashion, as I add new songs to my library. So I don't use the feature. But it might be great for you.
If I were in your situation, I would externally process the hard panned songs, and probably make them have a more mild pan. Maybe 15% off of center for the left sound and the same for the right sound. That way they wouldn't be "hard mono", but wouldn't be super far panned either. Then, whenever you play these songs *anywhere* they'd have the same sound. I'd also probably save the originals somewhere else and mark them so that I didn't play them in MC.
Just some thoughts.
Brian.