Custom Window/Overlay?

Guessing this isn't something the roll20 API allows access to, but thought it worth inquiring. Is there a way to create a custom pop-up box, similar to the initiative tracker? Interested in creating/seeing some kind of "party at a glance" window that can be viewed by GM-only, or all players. If this creates security issues, is there a way to have a Handout do something similar?

You can do almost all of that in a Handout.  The part you can't do is pop it up automatically.  You can whisper a button to individuals which they can click to pop it up.