I have plenty of experience with campden tablets, and it doesn't really "kill yeast". It, in conjunction with potassium sorbate, does inhibit yeast reproduction. That means in a ferment, it's really tough to stop a fermentation already in progress. It's possible, in winemaking, to allow a ferment to finish, then use k-meta (campden) and sorbate together to discourage fermentation to start up again when sweeteners are added. I think banking on campden to "kill" yeast would be a bad plan.
If you're kegging, though, I assume it'll be in a kegerator or freezer. Ale yeasts don't like to work at those temperatures, so in theory, you could sweeten it to taste and put it right in the kegerator and force carb. I'd still recommend lactose for sweetening- it sure is sweet enough. That one recipe around here for caramel cream ale with lactose is gawd-awful sweet.