Everybody seems to have missed this part in the OP:
This would fix problems of having to have special work-arounds for same-named artists
I don't think it was missed. It was commented on that it would probably not be realistic to expect that such a solution would be less complicated and easier to maintain compared to using a custom tag for the purpose.
Suppose it would be something like your mock-up:
Artist name, MusicBrainz artist id, and Country define an artist.
That would work if all tracks of your 'The Angels' example have all these tags populated exactly the same.
But realistically, there will be tracks that are not matched at MB and won't get an MB ID automatically.
Then MusicBee will split up that same artist as two different artists.
Now suppose one or more tracks don't have the Country tag populated. Or perhaps just written differently.
Now MusicBee will again split the same artist into different ones.
So using your three 'The Angels' artists as an example, you could now easily end up with MusicBee showing some 10 or more 'The Angels' artists.
It would be a lot of work to maintain and correct that.
And even for a unique artist in your library it would be a hassle. As soon as you have one of his tracks tagged with either a country or a MB ID, you will be forced to check all of that artist's tracks and make them all uniform.
So unless I am missing something in how you imagine this working, I don't believe it would be either useful, nor easier to use compared to just having a single custom tag, that you only have to maintain for your duplicate artist name artists.
But, in the future, with technology and databases progressing, this might perhaps be more realistic some day.