I have FO. Care to share the steps? Since WL uses it's own internal icons, I don't see how FO can override them, but would be interested to find out.
You have to do this in 2 parts
Part 1
-Launch FO
-Click Apps
-Scroll to the app whose icon you want to replace with a custom icon
-Click the circled arrow
-Select ICON
-Click Android Image Gallery (if you filed your preferred icon in your Gallery)
-Browse to the icon with which you want to replace the original and click it
-You'll return to FO and you'll see that your custom icon replaced the original icon
-Exit FO
***Note: Once you customize an app icon via Folder Organizer, it becomes the new "default" icon for the app wherever you
*you add it via the shortcut method below*, even in LPP, so you don't even need to replace the icon in LPP anymore when you add the app to your homescreen
(as an FO shortcut).
Part 2
-Go to WidgetLocker screen, make sure you are in Tools mode for WidgetLocker
-Long click an open space and select CUSTOM SLIDER
-Select the left/right/center dropdown arrow (wherever you want to add the customized app/icon you created in Part 1)
-Scroll down and select "SELECT SHORTCUT" (you may need to wait a while for the next screen, be patient)
-Scoll down and clcik FOLDER ORGANIZER ITEM
-Select APPS
-Scroll to the customized icon/app you created in Part 1 and click that item
Then Save
Voila!