I think it would be nice if it were possible to add a paragraph of custom text to a nation's description. 300-500 characters should probably be adequate. Considering that flags can be up to 10 KB, a few characters of text would be small by comparison (500 chars in unicode would be 1 KB, I think).
It might be worth making this a bonus, the same way custom nation titles are - you could make it available once a nation's population reaches a threshold value.
Edit: I should probably add that, if this is not possible, I will probably understand the technical reasons, but it seems to me a matter of adding a new database table, a little extra display code and, if the pop. check is used, some simple logic.