Looking for Ability to Upload External Libraries Through the UI, like Modules and Themes

I think this is a good long term goal. I'm going to refer you to the Libraries API folks, as they might have some suggestions about how this might be achieved.

The update manager in Drupal core is a nice thing and it would indeed be nice to support libraries with it.
BUT:

1. We're still in the phase where we're sorting out our APIs so that the module can actually fulfill its (primary) purpose and be useful for developers with modules that implement external libraries. We don't even have a UI yet whatsoever. So while this is definitely on the roadmap, this is very, very far away still.

2. I didn't follow the update manager getting in core and I have yet to hear of a case where it is being extended by a contribute module, so I have no clue whatsoever if it's even possible (though I'm assuming yes) and also how much effort it would be. So even if we had everything else done, I couldn't tell you how long it would take.

I'm leaving at active for now, though this is basically postponed on every other issue that's currently in the queue.

more in-depth discussion is here: #681034: Download Mechanism

While I considered this issue to be more about (eventually) using the upgrade manager for downloading libraries, where the other issue is more related to downloading the libraries programatically, but since this is very far off, I don't mind closing the issue.