Synchronize two displays of a view?
I got this working, and I didn't need Eva, Views Field, or Panels. I just needed good ol' fashioned Views.

I created a View with a Page and a Block.

Page displays images and takes contextual filters Term (of vocab Camp) and Date. 
When the value is not in the URL, a summary is displayed. 

Block displays Terms (of vocab Tags) and takes the same two contextual filters. When the filter value is not available, I choose Provide default value > Raw value from URL > and select relevant path component. 

I set the block to display on all pages, but on pages that don't pass it recognized arguments it just doesn't appear because it's empty. 

Voilá. So much simpler than I thought.