I actually have brined a Tri Tip, it was just one of those "oh look what's in the fridge, let's try this" kind of ideas lol
brined it with just apple cider, only filled it to where half the meat was covered, then flipped it after about 4 hours, for a total of 8 hours......didn't notice a flavor difference, but the meat did come out with a slightly amber looking color after it was done. Looked awesome.
I agree with everyone else, there's no need, nor have I noticed a difference when brining. I've also injected tri tips before and that does help introduce a slight flavor profile to it and also yielded some very tender (at medium / medium well) temp for the roast. I personally don't care for the medium rare that other tri tip cooks go for.
Long and short of it, don't waste the products to brine this great cut of meat. It's just not worth it......if anything, I like to substitute water in the pan for some apple cider with sliced up onion in it to add a little bit of aroma to the meat........does it work??? I dont know, but it still tastes great :)