I know you asked about liquid smoke, but to me nothing tastes like the real thing. This is why I love to cold smoke the raw hamburgers for a couple of hours before sous vide.
I also like doing this with steaks, wings, pork etc.
I use a small under counter refrigerator that I found used for $20. You want to use a smoke chamber that is cold since you will be smoking raw meat. I plug it in and get it cold first, then put the raw patties in on a wire rack. For the smoke I have a homemade cold smoke setup that uses pellets or small wood chunks (the flavor is up to you...apple, peach, mesquite, hickory etc.)
It uses a venturi / injector design to pull the smoke from the can of smoldering pellets/wood and delivers it to the refrigerator through a 1" pipe. I drilled a hole in the bottom corner of the refrigerator. Make sure you are VERY careful to miss any lines that are embedded in the walls of the refrigerator.
It is very efficient, smoking for hours on a couple cups of pellets. 2 hours seems to be plenty on burgers since they have a lot of surface area to absorb the smoke. I will do steaks a bit longer (depending on the thickness). The inside of the refrigerator gets a horrible stained brown color - the color of smoke! I just wipe it out when done and don't try to clean the smoke stain off it.
After smoking I vac seal them for sous viding. The flavor is very noticeable and very authentic. Yes it takes a bit of time, but if you are already going to the work of grinding your owne burger, sous viding and then searing, you already know the benefits and readily accept the work!
Refrigerator Style: http://4.bp.blogspot...efrigerator.jpg
Cold smoker setup: http://www.instructa...ocktail-Shaker/