For the flicker matching I ended up using GenArts Sapphire FlickerMatch (which I didn't know about when I posted to the list). It gave the result I needed in a small number of clicks.

Phil sent me an AEP with the sampleimage expression command in use but I didn't have much success with it. I spent more time than I had messing with it and since S_FlickerMatch worked I went the plug-in way.

But good to know more about the expression. Thanks for sending it Phil!