that's pretty damn cool, but I think I would get myself into trouble if the bartender just kept 'magically' coming around every time my glass got low in contents.
It'll never sell in Europe. They have their own way of telling if you are empty and want a refill that's worked for hundreds of years and don't cost a thing.
Glass mug...they can see if you are empty.
Ceramic mug: When you lay the mug on its side it's empty and you want another.
Ceramic mug with a pewter top: Leave the lid up if you want another. Other than that the normal position is down.
This way they don't bother you like they do in the States. They can check your status from across the room.