Multisite but for blogs in different hosting?

Hello! I’m wondering if you might help me with a question I have. I would like to build a community/portal for my clients. They will all have their blogs on their own hosting but I would like to be able to have them add a plugin to their blogs that would give them special access to plugins and themes that would only be available to those who are our clients. Ideally, I would like them to have this access through their own blog dashboard – rather than having to log in to a different site. Is this possible to do, do you think? Would one of your plugins help with that?

Thank you!