I think it's a cute, but rather strange idea to put flags on covers. It may work with Spain and America, which didn't have any significant border changes since romantic times, but how about 'flagging' Dvorák? Didn't Austria-Hungary came to an end after his death? Bit of an anachronism to assign him a flag of a state (Czechia - or Czech Republic) which didn't come into existence until after his death, even though we do consider his music to be typically Czech.
Or what about Franck? A French composer, born in Belgium, which was part of the Netherlands at the time of his birth...
And Schumann? Would he have gone by the East-German flag until the late 80's?
Flags might only really work with modern composers, who usually compose in an idiom so international that designating flags is rather useless. The flag thing is a paradox, altogether.