Hmmm, posting before thinking or testing, always stupid, but...
It looks to me like this can be done in PS7 using the inverted luminosity channel as a mask and a layer with Blend mode Luminosity? (Or some other blend mode).
If any of you gurus out there could verify this, it would be quite easy to build an action for it, wouldn't it?
(Looking for arguments not to update to
PS CS!

)