Hi Moose!
I'd simply recommend to be creative!
From my point of view, I would add a secondary screen if and only if the angle is different. If you're for instance making a handled camera and displaying it on a big screen with a medium resolution to keep performance nice and smooth. If you display it on another screen, you will have better perf keeping only one camera and using the same source.
If you're adding a secondary camera even if the texture resolution is low, you're still adding another render texture on top of the first one. The overhead cost of adding another screen that relies on the same texture on the other hand is quite low.
You can confirm that by putting a single camera and, I don't know, 5 or 10 screens... you should see a really low impact compared to more lower resolution screens + the same amount of cams.
Exemple stress test I did: one cam at 2048², one character, 12 screens, the average did not even drop two frames per sec and is around 198fps. 100fps in 4096²
As for the Monitor cam, my goal was never to replace the Window Camera, but simply to ground some camera/screens in the scene as an interactive/fun mechanic. I don't know what I could achieve with the monitor (since it is a vanilla feature and I'm not sure what I could and could not override).
But even tho you know way better than me what people do with MetaChat, I'm suspecting they might do something else than recording their monitor when they have their headset on
I'm not saying it's not interesting, just that the potential use for this purpose might be extremely niche. I prefer to stay focus on bringing new shaders and new features or performances optimization right now.
I could change my mind if suddenly dozens of people asked for that... but at the same time, I could argue that: why would you try to use a more complex setup ( several empties + plugins ), if the window camera does that for you natively.
Which makes me think of one alternate possibility: maybe the solution for a use case like that would be to grab the window camera texture instead as a potential source for a screen : )