I've never done such a thing, but I shouldn't think there'd be a problem. Koelsch yeast tends to fruitiness when fermented warm, but that's not out of place for a stout.
+1 to BOT (blow off tube) ... Kolsch plus a particularly malty wort will = BIG friggin kraeusen. I'd just make sure to try to keep it under 67˚-68˚F.
That being said... I could see it being delicious! I have some Kolsch yeast in the fridge at home, you may have given me my next big idea.
Koelsch yeast is excellent for many stouts and porters where you want it clean. I know a guy that ferments a fantastic Baltic Porter and RIS with koelsch yeast.