The idea is good but i don't think that Nintendo would even want that to happen.
Sharing a NNID between games, letting it record what you do or achieve and then letting you analyse all that in a single menu (accessible wherever) is probably what Nintendo is aiming for.
Even if Apple and Android would agree on having a Nintendo marketplace within their own systems, that would automatically reduce the exposure of Nintendo games because they wouldn't be visible from the get go. And what Nintendo wants is simplicity and exposure.
Nintendo could just create a unique NNID app that allows you to see your account and Nintendo's other offers (without being able to buy stuff) - an info channel if you will - that can be accessed via in game if you wanted.
But that probably would be too complicated?!
