i have not tried it, though i save some for starters. i think it would be fine but a few things i would consider (not sure how relevant) i would still boil it, which you have to melt it anyway, and second, i would dilute/concentrate until it is close to the same specific gravity that corn sugar would give you. that way you should have close to the same carbonation without exploding bottles or poor carbonation. keep good notes and let us know
I’m sure it would work, but you’d need roughly 2 L of 1.040 wort (assuming 75% apparent attenuation) to carbonate a five-gallon batch, and I’d worry about the flavor impact of diluting it that much.
I know you said you like the idea of doing, but I’ve gotta tell ya that after trying it 3 times, I wouldn’t do it again. I found it to be inexact and hard to control the carb level, and it really didn’t produce better beer than using sugar for priming.
Sean and Denny make great points here. While using 3rd runnings for carbonation may not be a great idea–a better one might be to use it for your 2 L yeast starter.
I commonly carbonate with wort. But I use leftover post boil wort. This way I don’t change the beer. I do agree that using the 3rd runnings may change the taste of the beer.