What I don't like (and I'm hoping there's a workaround for?) is the way galleries are only attached to single posts/pages (correct?)—I'd like to be able to manage galleries separately, and then add them to one or more posts/pages.
The way that WP-Juicebox currently works is that each gallery is associated with a single page or post and, if the Media Library is used as a source of images, then the images attached to the page or post containing the gallery will be displayed.
Unfortunately, there is no easy solution to change the way that WP-Juicebox works.
However, rather than try to separate galleries from their pages or posts, it would be possible (with knowledge of PHP and WordPress functionality) to change the way that WP-Juicebox displays Media Library images.
One possible solution might be to tag images in the Media Library (with a third-party plugin which allows such functionality as WordPress does not natively support this) and modify WP-Juicebox so that each gallery displays only images with specified tags. However, this would require a lot of work (and it would work only with a specific tagging plugin of your choice as all tagging plugins use different terms for their tags).
Could Juicebox be tweaked in some way to pull from FooGallery galleries, like it does from NextGEN?
It would certainly be possible to extend WP-Juicebox to use a new source of images (such as FooGallery) but an in-depth knowledge of how FooGallery works would probably be required (at least knowledge of how and where FooGallery stores images or image data) and I am not familiar with this plugin.
if I had my authors use Juicebox Builder, could we upload the files directly to the CDN, and then point the embed code in Wordpress to them?
You could certainly manually embed Juicebox galleries (created by JuiceboxBuilder) in WordPress pages or posts using the baseUrl method of embedding documented here.
Essentially, once you have created a gallery with JuiceboxBuilder-Pro, you would upload the complete gallery folder (not just the contents) to your web server and paste the baseUrl embedding code into the body of your WordPress post (ensuring that the method of entry is 'Text' rather than 'Visual'). It does not matter where on your web server you upload your gallery folder to as long as the two paths in the embedding code (the path to the 'juicebox.js' file and the baseUrl itself, pointing towards the gallery folder) are correct.
(Our photos are coming out of Lightroom, anyway...)
As you are already using Lightroom, maybe you'd like to use the Lightroom plugin (which comes bundled within the Juicebox download zip package) to create your galleries (rather than JuiceboxBuilder). This might be better suited to your current workflow. More information about the Lightroom plugin can be found here.
I don't know if my notes above will actually help but I hope they might at least clarify a few things or point you in the right direction.