Well.... If you keep your vent closed when smoking, say a chicken, the moisture from the poultry will hang around in the cabinet and condense on the ceiling, dissolving the smoke that has adhered to the surface of the smoker, which will then rain down on your food.
Not exactly liquid smoke but close...
I would think that one would have to duct the smoke into some sort of cooling apparatus which would cause the smoke to condense on the cool surface, and then you could catch that run off, and further distill it if necessary into liquid smoke.