If you just want to blank out, then use a second machine to feed the projector, I would try to turn signal down to black/very dark grey in V4 and then switch with a simple & cheap non-buffered HDMI or VGA switch to second machine. If both machines output the exact same resolution and refresh rate you might come away with a small glitch instead of a few seconds black. Depends alot on how your projector likes that. I remember having problems on some to detect pure black in such a case, at least when using oldskool VGA. Thus the dark grey.
About delay: delay sucks, but taking into account that sound travels only around 330 meter per second, a viewer that is 20 meters away from speaker already hears the sound ~60 ms delayed. One frame delay on 60 FPS is 16 ms, so if its about lipsync/audioreactive stuff in larger places this is a non issue.
If none of the above works for you and you don´t mind lugging around desktops you can use a Nvidia Quadro with genlock input to sync second source to first.
Would also work with older Quadro versions I guess.
@antokhio: Do you know what was to cause of burning down capture card? Internal failure sending out high voltage on signal lines? Or simply voltage difference in ground (aka happy little “sparks” when shielding touches plug…)