Nice-looking app, congrats! Hope you decide to bring it over to the juce world!
If it's of any interest, we're about to release the Tracktion marketplace website soon, which will essentially be an app-store for plugins (or any kind of audio app, I guess).
That will use a juce module to automatically handle the user registration/unlocking handshaking process, so should be really easy to integrate into a juce app. (This module will also be designed to allow it to be used with other websites too, so you could also use it as an easy way to roll your own web-store if you wanted to do that). I'll be releasing the module sometime in the next week..