I like the suggestion. It makes things clear for people, especially new, potential customers. It is super easy to make a mouse-over popup flag that says "on sale" or "Exclusive" or 'Clearance" it's simple HTML. Adding a key to the top or bottom would be even easier. I do not understand why there would be any pushback on such a simple suggestion.

 My Rendo-Space 
Do you know where your towel is?! I love Vogon Poetry. :P
DON'T PANIC!

Portland Pirate Festival Arrrr!