You should be able to define a new MC field called something like [Album Group]. Then you can set your main albums view to group by that tag instead of [Album].
But this gets tricky because any album that does not have a value for [Album Group] will all be grouped into one big group, since their values will all be blank.
So, you probably want to do this:
[Album Group Manual] = Field where you put Album Group information for those albums that need it.
[Album Group] = A calculated field. This field takes it's value from [Album Group Manual] if it has a value. If it does not a have a value, then it takes it's value from [Album]
That way [Album Group] always has a proper value, but you can override it at any time by typing a value into [Album Group Manual].
I believe this will work the way you want it to, but I'm not sure. You'll need to do some testing. Creating a few field is easy. Creating new views is easy.
Before you begin, think about what you want to see at what level. For example, when you see an album gruop like "Beethoven Piano Sonatas" and you click on it, what should you see next? The individual albums?
If that's what you want, then this gets weird because your album group for regular albums will end up showing you the album group (which is the album name) and then the album name (same name) on two different levels. Is that OK? If it's not, then I'm not sure what to recommend.
Perhaps you want a new view that is ONLY for album groups. That way you can have three levels (Album Group, Album, Songs) and it will be consistent.
I hope this gives you some good info to think about and experiment with.
Brian.