When we interview someone on our show we need to have them sign a release form.

We would like this feature to be added to the Join Call page as a required checkbox before the "Join Call" button is active.

There would be a link to read the terms in a popup.

The terms could be updated by us (your licensed customers) - Or perhaps the terms could be standardized and work for anyone so we don't have to upload them.

As a quick fix I am considering sending people to a custom webpage first and then redirect them over to the join call page --- but ideally its all integrated thru Vmix.

I think your idea of a passthru page is the way to go. A general release form is problematic because of different legal requirements in different parts of the world, and there are many circumstances where you will not ask for, or require, a release for the guest to participate.