Give me a break....
You won't kill the yeast even if you drop your 2 cups of boiling sugar into the bottling bucket. It's a ridiculous notion from the start. It doesn't matter if you add it boiling, warm or ice cold. The falling through the air space will lower the temps anyway, as will touching any sanitizer dregs/foam in the bottom of the bucket (which you should have). Bu the the point is moot because seconds from when it gets in the bucket, 5 gallons of room temp beers is going to be crashing right into that, heck you should even have your beer already flowing in there to begin with- who's going to win?
The solution's not going to be hot for long.
And even if it were going to kill the yeast
it's only going to kill the first yeast flowing into the bucket anyway...but again as soon as the solution comes into contact with the priming solution, it's going to be cooled, so it's not going to be doing much harm to the yeast anyway.
There's going to be a ton of yeast and beer in the still in the bucket long before the deadly hot yeast killing priming solution is still going to be remotely warm.....
It really doesn't matter how you add it. It's all going to mixed properly and there will be plenty of yeast to do the job-regardless if any was harmed in the bottling of the beer.
And if you're so concerned about it, just work cooling time (even just on the counter) into your bottling proccess. With my system I usually have 1 case of bottles still yet to santitize when I pull my solution off the stove. A couple minutes later it's already pretty cool anyway.
I talk about it in my thread http://www.homebrewtalk.com/f35/bottling-tips-homebrewer-94812/
But the idea of 2 cups of boiling priming sugar being in any way harmful to the 5 gallons
of beer is just ridiculous, and not very well thought out. It's not like we're talking 5 gallons of boiling liquid crashing into 2 cups of cool beer.....it's just the opposite- the 2 cups doesn't have a chance is this fist fight.