Hey that page has a pic of the Minicade I was talking about:
Matt, are you proposing a see-thru LCD?
Yes, if it would work. I may be wrong, but I think LCDs are more-or-less transparent with no current applied. The more current, the more the crystals are twisted, the less light gets through the two polarized panels, and the darker it gets. You decase it, mount it in front of the vector monitor, and you should be able to see the vectors through the projected overlay.
(Here is one of those DIY projectors I was talking about.)
http://www.instructables.com/id/S1A1L23FGC213NR/You could also use an LCD panel for overhead projectors, I suppose, but all those I've seen are expensive, small, and bulky, so you'd have to take the case off anyway.
Or, as is entirely possible, you won't be able to see much of anything though a decased monitor, and the half-silvered mirror is the only viable option for such an undertaking. Takes up a lot more space though, and one of the images would have to be reversed. Not sure how you do that in vmame, short of switching x-deflection wires to the vector monitor... I'll put this on my long list of things to mess with.