Creator of Mario Strikers Charged tells French press that Friend Codes are tied to Miis.
Mario Strikers Charged
In an interview with the French website Gamers.fr, Justin Dowsedell of Next Level Games mentions that the Friend Codes generated when you use a Mii to go online in Super Mario Strikers are used in every online Wii game from here on out. What this presumably means is that, say, Battalion Wars II will not require you to generate a new code as long as you're using the same Mii. (Update with better English translation from our beloved Platformer Poik007:)
Gamers.fr: And for Mii codes, how is it going?
NLG / Justin Dowdeswell, Producer: Every Mii created generates a Mii code that will never change after its creation. Therefore this code will be used for all other online games in the future.
While far from confirmed, this snippet from a Wii developer is the most official source on the matter so far and makes the most sense for Nintendo to adopt.