I would be interested in seeing a solution to this.

What is the context of what you are trying to do.

I can't imagine a permissions database  interface that will be more
intuitive than doing it directly from SharePoint.

We've considered using a global Role Provider database for multiple
applications on campus, since we have several enterprise applications
that could utilize one. If we ever did, what you are talking about might
be very useful.
Every Site collection is going to have a different
sharepoint group, even though they may be called by the same name,
security is scoped to within a site collection.