Yes, this technology currently based on Elgg (you can see “Powered by Elgg” on right bottom corner of any plugin page). Elgg is pretty powerful GPL social networking platform, and works like a charm =)
It does not make GRA4 plugin bad, does it ? 😉
The point here is that you must say that it’s a layer that AFAIK redirects to your Elgg installation.
Elgg is released under GPL license ver.2 so you must do that.
I presume that you’re trying to build some kind of (paid) service in the future.
For now, what you are doing is something that might pose a ridiculously high security breach in each WP it is installed.
That said, you and your friend are very skilled programmers. 😉
The interface IS Elgg.
Even GRA4.com is a out-of-the-box Elgg installation!
Why don’t you try to install Elgg and see for yourself?
Why not choosing BBpress or IntenseDebate? They’re made BY WordPress…