This idea of a download play updating systems is REALLY intriguing though! I pretend to know a lot, but really I have a very limited understanding of how patches work on the 3DS.
I don't think it's ever been done, so it might not actually be possible. It's more of a "Well, if you need to get the patch in a local environment, this might be one way to do it". The same guy here saying updates are required for local play also said "Streetpass can give you the update as well", so maybe that's another way that's actually been done before. I also pretend to know a lot. :X