Alas no, the way actlabs guns work means they need access to the signal that actually ends up on the screen, not just a copy.... This is because they alter the signal when you pull the trigger (they turn the whole screen completely white momentarily).
There's a company who have recently released a lightgun for the xbox/ps2 and are thinking about releasing it for the PC - someone from the company posted here on a thread asking about mame. E-real games or something. Anyway, if they make a pc version, I imagine it would work with your setup as it uses a camera you setup at the top of your monitor. No white flashes either....