Cheers guys.
I've just thought of a practical reason why what I mean isn't possible right now.
When you hold down a scene button to start scene-locking parameters, the trig buttons switch so you can select a scene. To do what I'm talking about you would need to have access to the sequencer trigs as you are holding down the scene button.
I suppose I can do a complete shift in sounds by having two different loops on one sample file, and then scene-locking the 'start' parameter. But that trick is probably old news by now!
EDIT: My understanding of that sentence in the manual is that scene-locks override p-locks.