I had tried searching before posting the same question and didn’t see your post at first.
Wondering if you found a way to do this?
It was a feature of my Matrox APVe that I really found useful.
BM emphatically claims that Matrox video cards are not compatible with Decklink so my Decklink system was setup without one.
I had assumed the incompatibilities might be due to conflicts of the same feature we are looking for.
Matrox used a DirectShow filter that automatically came up in the graph whenever a video was being rendered. The only caveats where that some colorspaces were not valid for the Matrox filter.
With that I was able to use MPC, VLC, WinAmp, Media Player, Nuendo, and a few other players/applications to output the video to either an HD or SD monitor (Composite, S-Video, Component) simultaneously.
Looking to do the same with a Decklink HD Extreme through the Y/C outputs.