icloud and external drives

I have years of photos and videos (around 700GB) and I have always stored the library on an external drive and used iPhotos - and now Photos. The external disc is then backed up on a couple of other drives for safekeeping.

I now wish to use iCloud but I still want to keep the primary library locally and have it backed up on iCloud so that

(1) I can access all my photos from my iPhone or iPad etc and

(2) for added safekeeping.

I am told that I can only do this if I store the library on my internal drive.

I have a MacBook Pro and I dont want to fill up most of the hard drive with my photo library as I also have a large iTunes library etc.

Does anyone have any suggestions or is Apple going to change this so that you can back up from an external drive to iCloud?

Thanks

Armen