I'm not assuming when the formalisation happened. I'm saying that it's harder to get everyone to agree on how the orthography is supposed to be, when 2+ governments and populations associated with them are forcing distinctions even when there's none.

You're right that it is not impossible however, and your historical example shows it. Historically Lithuanian is the exception that proves the rule because

• the local population didn't see themselves as Prussians or Russians, but as Lithuanians, so there was a community even across borders; and
• neither Prussia nor Imperial Russia were backing specific varieties of Lithuanian. They were backing German and Russian instead.

And nowadays it's simply not an exception. (I was referring mostly to modern times.)

Instead, books in Latin script were printed in Prussia and distributed in Russia illegally. A handful of people like J. Basanavičius and V. Kudirka ended up in charge of printing most of those books and it made it easy to set language standards. Achieving such a monopoly with a bigger language would be much more difficult.

That's a great tidbit of info, and it's related to what I'm saying: those Lithuanian speakers in Russia only accepted the books as suitable for their language, even if they were printed in Prussia, because they didn't see it as coming from "those other guys".

[Thank you for the info, by the way! Across the whole comment, not just that paragraph.]