It's been a while, but I've done briskets, as well as butts, straight from the freezer, without thawing, to the smoker a number of times without any issues or problems.
Obviously, you need to add some additional time to your normal anticipated cook time. I don't recall exactly what I did, but I think I added 25%. After about 3 hours, check the brisket and see if it's thawed enough to add a temp probe, and if it has, do so. If not, check it after another hour.
You should be fine.