Yeah, this MUST be a size issue since SSBB, Boom Blox 2 and many other games including Dewy's Adventure for cripes sake allow you to download extra levels (usually user created but EA is making new levels for BB2 downloadable).
If the levels are beyonds Nintendo's limits for file size (which is likely) then the only way I can see around that is if Activision supplies it's own 2gb 'CoD' SD card with every game. The CoD SD Card would have an extra level on it and unlock the downloadable material. That would be a simple solution that Nintendo might accept. An extra $5 on the purchase price ought to cover costs for Activision who would then make extra money selling more levels.







