I've never done sous vide - how much of this liquid is left after cooking? You need a fair amount of water-based liquid to make an emulsion.
I suggest you try it and see what happens
Let's say you have 1/4 cup of liquid. You could sear the ribs in a pan after the sous vide cooking, then deglaze the browned bits with the liquid; melt 1/4 cup butter in the liquid, then whisk in 1/2-1 tsp. mustard. This would give you about 1/2 cup of an emulsion. Taste and let us know what you think