I didn't try John's Solo command, but it was talking
explicitly about 'elements' so I'm guessing it might not handle
Layers (another one of his commands was documented as not handling
sublayers or something, so I'm being a bit presumtuous but...)
...but I found the answer myself!
It looks like Layer visibility is animatable, so the its
visibility flag is part of its Frames object.
ie, this code should hide all the top layers...