Ever heard of a Taco Burger? A local (Taco Inn) place use to (still?) sells taco meat, cheese, lettice etc on a bun. Kind of different if a bit odd, but nothing too special.
There's a regional chain in the inland northwest called Taco Time that carries a taco burger like what you describe. Taco meat, lettuce, tomato, and IIRC Thousand Island dressing on a bun.
The Taco John’s chain offers a taco burger. http://www.tacojohns.com/food1-taco5-Taco_Burger.asp
Beware as it looks nothing like the picture when you order one. Of course, what fast food does?
The bun is usually hard too, like they microwaved it before they slapped on the toppings. I get a craving once or twice a year for their taco burger as it was in the past. I’m disappointed every time but I still keep on trying.