yeah man i did this very recently. i added the potassium sorbate and the campden right to the keg and racked onto the chemicals. i think the only difference from if you did it in a secondary is you will have to vent the keg a few times.
after everything is in the keg, i just let it sit as is for 24 hours. then i hit it with co2, shook the keg, vented, and repeated a bunch of times over the next day or so to let the gasses from the chemicals escape. i also added a can of concentrate (the whole of point of me using the campden and sorbate.) it worked for me- within a few weeks after carbing it, the cider was drinkable. and the concentrate improved the taste a lot.
its probably not the textbook way to do it, but it seems to work fine.
You must love this country more than I love a cold beer on a hot Christmas morning. - Homer S.
And how is education supposed to make me feel smarter? Besides, every time I learn something new, it pushes some old stuff out of my brain. Remember when I took that home winemaking course, and I forgot how to drive?- Homer S.