Yes, it does exist and is currently being tested internally at the client we initially built this for.
This is not an app you will just be able to install and be done with, we'll have to do some implementations from our side in order to make it work.
So, requirements so far:
Implementation work from our side
Some license (not specified yet)
We were hoping to have a video demo ready last week, but we did not have the time i'm afraid. Untill then, and if you are still interested, send me a mail at email@example.com and I will arrange for a demo for you
I will just chime in and say that I am moving from X-Cart to Shopify and X-Cart has this ability. THIS IS A VERY, VERY important ability, because many times a user will say that a coupon doesn't work, they can't see any discounts, or a feature is not working for them and it helps to login as the user to see exactly what they are seeing.