Getting an Alpha channel out of a Mac for live switching
 

I'm in a situation where the client would like to use their Mac as an overlay CG for live switched programming. Using an Extron DVI scan convertor or a FireWire DV>Analog convertor works OK but it would be nice to send an alpha key signal to the switcher and not worry about the luma key knob on the switcher panel getting bumped. BlackMagic Designs has a few cards that can do this but it seems that the cards are limited to using a very basic software program to send the graphics out thru their hardware. Ideally it would be nice if there was a piece of hardware that could be adjusted for the black level that creates the alpha key. In addition the client wants to use laptops.