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As I understand it, a riving knife prevents kickback by keeping the workpiece from pinching or being pushed into the back of the blade.
Pinching the blade is unlikely to happen with non-through cuts, which all dado cuts are, but the workpiece could still theoretically be pushed askew and contact the back of the blade.
Would a riving knife with the same thickness as the kerf of the dado stack prevent this? Or are non-through cuts just not as dependent on a riving knife for safety?
I think the major danger that riving knives address is binding on the blade from the kerf closing. This is a particular risk with solid wood where there may be stored internal stresses (e.g. when the wood is said to be case hardened) which are released when it is cut, where the wood can twist and actually close the kerf tight in the worst cases.
Since this is not a risk when doing dado cuts for the reason you specify — not a through-cut — I don't see that a kind of riving knife, sized to match the dado stack, would be of any benefit.
There is a real danger from kickback here and that is inherent to doing a dado cut on the tablesaw in the first place. You might not be aware, it's impossible to do this in other parts of the world because there the saw designs don't permit a dado stack to be fitted, because of their inherent danger.
Other methods for forming the dado should be always be preferred, e.g. by using a plunge router working within a template that is clamped firmly to the workpiece.
