How cold do you think you can keep the inside of the "cold" smoker? I wouldn't do this if you're not sure you can keep the inside below 40... Too many potential food safety issues IMHO with the amount of time it would both be above 40 and then how long it will take to get above 130-140 once you toss it in the oven or sous vide. The general rule of thumb is "no more than 4 hours 40 to 140", because you can get pretty rapid microbial growth in between...
That said if you can keep the smoker below 40, I see no reason not to do it.
If you *can't*, that doesn't mean you shouldn't play around though. I'd see if you can get your smoker down to <200 or perhaps at least <225. Put the brisket into the freezer for 30 minutes or so before you put it on the grill. The cold temp of the surface will help it with smoke uptake and keep it from rising in temp as quickly. Go maybe 2 1/2 or 3 hours in the [hot] smoke, then go sous vide or oven.
Sous vide on a brisket can do some interesting things, I'd think. I've never done brisket per se, but I've done both tri tip and baby back ribs. Tri tip as most know is pretty tough, but 24 hours at 131F and it's tender as filet. Ribs I do 24 hours at 145 degrees, and they come out fall off the bone at medium doneness. I've done sous vide short ribs at 131F for 48 hours that were medium rare and could pretty much be cut with a fork.
If you're going to sous vide, I recommend not trying to make something even remotely similar to the temp/texture you get from a smoker or braise. If you're going to go for that, you smoke it or braise it. The thing with sous vide is that you can do things that can't be done with traditional cooking methods, like a brisket that's cooked to medium rare or medium and yet is perfectly tender.
Serious Eats has an article on it:
https://www.seriouseats.com/sous-vide-barbecue-smoked-bbq-brisket-texas-recipe
BTW my first sous vide experiment before buying a circulator was my Ranco temp controller powering two "coffee mug" electric heat elements, and recirculating using my brewing pump and brew kettle. I only went 12 hours as I didn't want to go overnight with such a DIY setup, and did tri tip at I think 131. Then seared hot on the grill. It worked swimmingly [pun intended]. I used to have a video of that setup on YouTube, but deleted it a few years ago. But it worked! So I'm sure a Grainfather can do it.
