Well... I've finally got something substantive (not what I've really been after, though, because my screen hasn't turned blue since I borrowed the camera), and it was some trick capturing it, because it happens for only a split-second after the gear wheel stops spinning; I lost count of how many times I booted and rebooted before I timed it just right.
This is a shot of what I described in my post #9500 in this thread, but I've now determined that it happens
only when I'm booting from a Snow Leopard volume into a Leopard volume.
It doesn't happen booting from SL to SL, from L to SL, or from L to L...only from SL to L.
It looks like a kernel panic, but it comes and goes without doing any apparent damage.
Does it mean anything to anybody?
Thanks.