In case someone else has this issue, here's how I've solved it.
On my pop-up panel, I have an empty background widget with collider that fills the screen and contains the selection widget. The selection widget works as normal, and if I click outside of it I hit the collider of the background widget. I have to deactivate all other components when it pops up though. I have a little SetEnabled() script that deactivates boxcolliders that I apply on the root containers for my various panels.
I think it'd be nice and sensible to have a 'pop-up' widget type that automates this as it's a common feature. It would be easy to create a prefab that has the background widget and actual component but the deactivation of everything else might be tricky. If there's a way to 'z-sort' controls, this deactivating step wouldn't be needed making the whole thing even easier. A 'pop-up' script would add a full-screen widget and a widget within set up to capture the outside box events.