Mornin' pigisgood.
Sauce on the side is only in MIM contests,and they are pork only.
The Tx folks don't allow anything brushed onto the meat for turnins.
The thoughts are that if it is cooked correctly,it shouldn't need sauce.
At home,use what you like.
A popular choice is to defat the drippings[although a little fat adds flavor] that collect while the brisket is holding in the foil.
Add a little bbq sauce of your choice,to make a thin sauce with a flavor that suits you.
If it is too salty,you can always cut it with a little apple juice.
As suggested,a little losalt beef stock is handy to work with.
A touch of cayenne will add a little back heat.
Seems a bunch of folks out there use Cookshack mild sauce,thinned with drippin's.
The goal ,for me,is something that tastes and eats like BEEF brisket and a little moisture can be added to keep from drying out after slicing/chopping and accents the flavor.
Hope this helps a little.