Well first, so you will know for your next brisket, there is not set Internal Temp when brisket is done. There is a point when at average EVERY brisket is past being done...many call that 205. But a BRISKET is done when a probe goes through the flat like Butter.
Next, you have quite a poser here because 12 hours is far away. I would not hold it that long unless... no, I just would not hold it that long.... someone may disagree.
My theory is, when you said "it looks done" at 189, well, with respect my new brethren, it may not be. Sure it is edible, but NOT what all it should be.
Here is what I would do. I would wrap it TIGHT in plastic wrap. By tight I mean several wraps one way. Several wraps the other, very tight until the meat is SEALED completely. Now you have a sealed brisket and no drying will occur.. if you do it right.
I would then pop it into the freezer for and hour and a half then pull it out and place in the refrigerator.
About 3-4 hours before your company gets there (I don't know if this is a packer or just a flat) place it on a cookie sheet or pan and turn the oven on 230. Pop the whole thing in there and let it warm up until the plastic loosen and forms a bubble. By then you have rewarmed the brisket.
IMPORTANT, let the brisket rest on the counter until that plastic shrinks back (pulling the juice in) and you can hold it comfortably in your hand. Cut the plastic, cuts the brisket cross grain and serve.
Certified KCBS Judge