How to show a center extraction overlay in RV? Mask in from the left and right as well as top and bottom

I need to do not just an aspect ratio mask overlay in RV, but I want to see the deliverable aspect center extraction area as defined by a show’s DI delivery specs. How do I do this? There only seems to be an aspect ratio overlay available in RV. I’m using RV supplied from Flow, not OpenRV, though I suppose it would be the same.
Is there a way to use an image as an overlay? Or is there a way to enable more than a simple mask? Please advise. This seems like a fundamentally basic feature.

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