I found a way to get this already today (in 4.5.1.1): In Preferences->Contact Sheet->Thumbnails, I set "Labels" to "Name + 1 Label" and "Label 1" to "{firm}". That way, below the file name for each image, I get the Firmware info which will tell me if the image has been edited or not.
Not as elegant as an icon to indicate edited images, but a lot better that nothing if you ask me!
You can use code replacements along with the {firm} variable to get even closer to an EDIT icon...
First, change the text for "Label 1" from "{firm}" to "\{firm}\"
Next, create a Code Replacement File with entries such as:
Ver.1.00
Capture NX 1.0.1 W EDITED
Now, it should show "EDITED" only with photos that have been edited.
Important notes:
1 - You'll need to make sure the code replacement text exactly matches the values from your camera and your Capture NX version.
2 - In the code replacment file, after the "Ver.1.00" you must include a tab and then a space. There must be a tab before the "EDITED" text.
3 - You should include additional code replacement entries for any other text you have in your {firm} variables
4 - Any {firm} without code replacements will not be "translated". For example, if {firm} is "Ver.1.01" it would show as \Ver.1.01\
I hope this helps.
-Ian