Ask HN: Gmail add-on to show emails I've sent where I'm waiting for reply?

Boomerang can do this for new emails, but it can't do it for ones I sent before I installed Boomerang, or where I forgot to tick to resurface if no-reply. Seems like it might be a useful thing, perhaps a useful tool for salespeople too. Ideal feature: I load it up on my gmail, and it creates a folder called "waiting for reply" and all the emails I sent with no reply go there. Does this exist? Would boomerang do it as a feature? Do others think this would be useful?