Splitting the game view - part camera, part opaque navigational panel at runtime

okay, so you do this at the gameobject level, but is there a way to do this from the global level? would you have to use a second camera for the 2d panel?

Yes, typically you'd use a separate camera for each discrete area of the screen that you want to render to.

Is there a way to specify this by percentage? What if you don't know the target screen size?

"Screen position out of view frustrum"

Regarding your question about percentage, that's essentially what the normalized viewport rect does: (0, 0) represents one corner of the screen and (1, 1) represents the opposite corner. Regarding your second comment, I assume this is an error that you're receiving? If so, it's hard to say what might be causing the error without more information.