The "Finland Scheme" is long outdated. The wiki page you cite has been written 12 years ago, without updates since.

The discussion about traffic_sign:id was very broad. The situation for destination signs is a lot simpler. Any new scheme can be extended throughout the whole of Europe without problems - and the existing scheme is almost capable of it, with just some minor tweaks and extensions. 

Please write a request for comments in the international tagging section of the forums.