If you water it quickly enough and deeply enough, the grass may survive. Many stump killers/removers are potassium nitrate, which can be flushed below the root zone.

But it depends--we'd need the list of ingredients to give you decent advice. And in any case, time is of the essence. Start flushing now, three hours in is already a long time as most things will leaf absorb.

Wouldn't potassium nitrate basically be fertilizer in a different form?

Yep, but it's in pure form as a salt. Pure enough to kill growth on the stump and poison any life left in the tree. Dilution below the root systems of the grass is required. :-)

In very dilute form, it could be used as a quite nice fertilizer and a supply of nitrogen and potassium in readily and instantly available forms.

I don't usually recommend it in my fertilizer mixes because it's expensive in terms of price per pound and nutrient unit, and only 13% nitrogen. Urea as a nitrogen source is far cheaper with 46% N, and potassium sulfate is cheaper and easier to find as a potassium source (and also brings some sulfur to the table).

For the rare plant that has a sensitivity to urea or ammoniacal nitrogen, though, it can be of use when applied judiciously. The K levels are very high, though, at 45%, so overloading the soil is a real possibility with this stuff.