The Nxt ID can be this UID, right?
But I agree, we need some sort of standard (or a recommendation, at least).
Your app uses a particular address as I understand, to create this "mini-google", right?
But what if there are several apps that use user accounts? Encrypted messages, plain messages, reputation system, receipts, what not.
EDIT:
And even with your app you already have a problem of type and need to create those tags.