I dissolve the gelatin in about 0.25 cups of water and heat it until it turns clear (160F or so). Then, I just pop the top open and pour the heated gelatin and water combination in, then put the top back on and blow it off a couple times to purge. Nothing fancy, and seems to work just fine.
There is a brulosophy experiment on kegging method (and how oxygen proof you try to be). It lends some insight into how paranoid you should be about it, though I don't think they aged the stuff very long. Then again, most everyone seems concerned with it for hoppy stuff which wouldn't be around long anyway.
As a side note, for those that do all the closed transfer stuff to the kegs, are you back filling the fermentor with CO2 as you transfer out of it? If not, then all the junk done on the keg end seems largely in vain since the fermenter is going to be swirling with mostly normal air all over the top of the liquid.