ALPHA CHANNEL
Don,
I started to answer your question and realised you'd answered if for yourself.
<g> I think you got it exactly, record the alpha matte to tape and use the GPI
to trigger the Preroll or however you have it set up. You could use the PAR to
record the alpha matte and play it back in realtime at what ever point you
specify. Should work very nicely, and is very useful because you don't limit
yourself to high contrast "flying logos" etc, and you don't have to worry about
"super black".
Always nice to see you excited about a new use for your tools. <g>
-JE