You can definitely do keto, but it might not be especially high fat for a while--most bariatric nutritionists seem to favor a low-fat, low-carb diet for new patients.
And very few people get through the first few weeks without something artificial. (I relied on sugar-free jello, sugar-free popsicles, and SO MANY PROTEIN SHAKES.) If you're really dedicated, though, you could probably minimize most of that. For instance, Eggface suggests "double milk" (1 cup Milk mixed with 1/3 cup Nonfat Dry Milk Powder = 16 grams of protein per 8 oz. serving) as one option.
Most plans won't force you to eat beans or fruit or high-carb veggies, though; the focus is very much on protein sources, mostly animal-based ones.
If you're worried about dumping from high-fat keto, you might consider discussing the option of the sleeve instead of the bypass with your surgical team. (There are people with sleeves who experience dumping, but it's pretty rare.) But like Matt said, not everyone who goes bypass has a dumping problem from fat, either.