Local backup of iCloud Photo Library
@Elliott: Do you have the option of creating regular backups of your external disks? If so, you could symlink your photo library to the external source, backup that source however often you like, all the while using your full res photos instead of the low res versions. The photos would not be stored on your Mac hard drive (thus freeing up that space), but would be stored on backed-up external drives instead.

I've successfully done this before using both iPhoto and iTunes (among other things), and now regularly move all my important things to external drives for safe keeping. Backups of your backups is key, but drive space is cheap!

Let me know if I can be of further assistance.