I cannot magically template already recorded and pre-mixed projects, so I guess there is no solution possible yet -_-
and it also makes little sense to use templates, because I usually choose plugins according to what it sounds like, not what instrument/mic I use, so again no good solution in my opinion :P
I am clearly not understanding what you want to achieve as it seems you want to use the SAME mix from different projects (then surely dragging in new audio to an existing template is a good start - even if it is from one project to another if it is already recorded) but you also want to choose different plug ins and mixes, which is a completely different mix isn't it?
I am not understanding how importing a mix scene from another project will help as surely the result of importing a mix scene will override your plugin settings and give you the same results as importing you audio in to another project with the same mix scene???
I am sure it is my lack of understanding in what you are trying to achieve (and brconflicts example about changing a compressor setting across projects makes sense)
So are you in fact looking for a solution where you create a mix and then when you tweak it - it gets changed across multiple projects?
I tend to mix studio projects so have not encountered this limitation as I use (and re-use templates) but I can see the use if mixing a live project
Here's an idea...
Could you mix all of the songs within the same sonar project - IE put the different songs at different points in the timeline?
You could then create a mix recall for each song within the same project and export each song by selecting a time range before export?