Listing all subdomains where user created blog posts

Have had multi-site up and running for a while now, with hundreds of domains and user created subdomains on a single install. Also have the global site search plugin up and working great for global multi-site queries.

Here's my problem. Suppose I have domain1.com. It is shared among 100 users. These 100 users create subdomains like sub1.domain1.com, sub2.domain.com, sub3.domain.com, etc, and they create blog posts on these subdomains. All of the subdomains are shared, everything we do is shared.

User1 creates a post only on sub1.domain1.com and sub2.domain1.com. He does NOT create a post on sub3.domain1.com. Some other user created content on that one. I need to allow User1 to press a button and get a list ONLY of the subdomains for which he created content for domain1.com. When he presses this button (or runs this query), he will get back a simple list that says, sub1.domain1.com, sub2.domain1.com. Stop. Do not list sub3.domain1.com because User1 did not post content there.

My engineer says multi-site does not allow such a query, but especially combined with the global posts plugin, it seems to me that such data exists somewhere and this is possible. All I'm looking for is the syntax: For all subdomains of domain1.com, where User1 created a blog post, list these subdomains.

Of course, what I'd like for User1 applies to other users as well.