Hello William and Paul,
Yes, I've VI Pro (the old one, with 8 slots) and I often use this tecnic, specially with strings.
I've tryed and actually it perfectly works with organ too, creating an effect that I don't think it exist in reality: a slot "cross fade" between 2 stops or registers. I'll ask to my friend organist. VERY INTERESTING!
What I'm not able to do (I'll go back to the VI Pro manual) is to shift, or switch, between slot 1 to slot 2 (in vertical).
From 1a to 1b I do it in Hyper Editor (Logic). but how to move in vertical? I'll study it.
Anyway, very usefull tip! I'll let you know If I discover some other thing. 
Thanks a lot and let's go on!
Roberto