You can filter a View Scheme in MC12 based on a Playlist (or exclude tracks based on a playlist).
1. Make a new View Scheme (right click on Audio and choose Add View Scheme). You could also edit an existing view scheme.
2. Give it a good name, and set up your panes/tree children in Step 1 if desired.
3. Under Step 4 click the Arrow and choose the Wizard.
4. In the Filter Wizard add a new rule if needed.
5. Choose Playlist on the Left.
6. Choose Is Not Any in the Middle. (This will exclude tracks that match the playlist, or lists, you choose in the next step.)
7. Choose your Playlist from the drop down on the right.
8. Click OK to save your filter and then OK again to save the changes to your new View Scheme.
You can do the same exact thing to filter your existing Playing Now list, if you wanted I suppose. A more permanent View Scheme would probably be a better option IMHO. But, if you really wanted, you could just switch to the Playing Now area and use the Arrow on the Search Bar and follow steps 3-8 above. That will filter your "list" in Playing Now, and work, but it is easily eliminated by clicking the X in that search bar (and it isn't really using the capabilities of MC12 to their fullest).