My curiosity got the best of me so I looked into adding support for the new library. I was initially planning to wait for the stable release for fear it would break Metro users not yet on the beta, but it turns out that fear was unwarranted! A fix is possible that supports both the stable and beta releases! This release also includes a couple of layout bugs I found while looking around the client. Check out out the full changelog below!

Download Metro - 4.4 (Direct)[github.com]

Changelog[github.com]

Edit:
Getting a few questions in the comments about what this update means. Without it, if one uses the Steam Library beta release with Metro 4.3.1 installed you'll get a blank screen with some strange bits in the upper-left corner. If you're on the beta release and have 4.4 installed, you'll notice that it adds support for the new Steam library as-is with some additional layout fixes. If you're on the stable release, you should only notice the layout fixes. This update won't enable the new library automatically.

In regards to skinning the new library, I'm reading that it's possible by overwriting default Steam files. That's pretty dang cool, but I have no plans to release any Metro updates utilizing this method. I'm hoping that in the future Valve will add a skin system/selector similar to what Steam supports now. Until then, we've got a bit of a hybrid thing going on!