Demiglace substitute? BTB+gelatin?

I’m doing a sous-vide brisket recipe which calls for demiglace, 1 tbsp. This goes in the bag with the brisket for 72 hrs.

Any thoughts about using a similar amount of Better Than Boullion along with a packet of powdered gelatin? Seems like a pretty similar flavor. I can always adjust salt to taste, so I’m not too worried about that.