If the cameras picked up the rest of the ride, why wouldn't it pick up the HHG? I've got tapes of the Mansion from the early 80's taken on a video camera that picked up almost nothing in the dark, and the HHG's showed up bright and clear. My guess would be that they either put too much lighting in the little hallway, and the light caused a reflection too strong for the HHG's to shine through, or it was an intentional leave-out. Not necessarily because the effect wasn't done, but because that was the final gag, and it was most effective as a surprise. So they wanted to keep it as such for as long as possible, and obscured the effect.

I think they just wanted to keep that last gag a secret, though that doesn't mean that there wasn't another version of the effect there at the time. I wonder if any of the projections, be they film or slides or whatever, have shown up anywhere? (I keep picturing the whispier spirits out of the kids book/45 rpm record set, actually...am I in the ballpark?)

-Adam-

Edit: Because I can't bloody read (and asked if they were animated, and that's one of the main points of the first post...d'oh)