With what argument? (Not that I wouldn't want to abolish them myself, but what is the rationale in your system?)

Or institute a similar EU-wide rule where if a country wants to have subnational constituencies they have to use the same Penrose + country-wide scheme as is used in the EU as a whole.

But a national level Penrose (apportitioning a number of seats as given by Penrose for the whole country), instead of EU level? With that, you are strenghtening the national character of the elections further.

_{*Lunatic*, n.
One whose delusions are out of fashion.}