I use Xylitol to backsweeten my ciders and haven't had any complaints from anyone. It's a non-fermentable sugar. It's sounds chemical and unnatural but it is completely natural. You should be able to pick some up at the sweetener aisle of your local supermarket, just look at the ingredients on everything in the specialty sweetner area. The stuff I have is actually cut with malto-dextrine, which is just a filler, so perfectly fine.
There are other non-fermentable sugars you could use as well, but I've had the best success with this one.
If you don't want to backsweeten with a non-fermentable, you'll need to pastuerize to kill the yeast. Here's a link from cider forums on stovetop pastuerizing in bottles. I don't see a good application of this for kegging, but the trick would be to get the liquid in the 150-160 range for 15-25 minutes to kill the yeast. There's lots of talk about cold-crashing, which if you have a kegerator, might be your solution.
No magic bullet to solve your problem, but maybe I've pointed you at some possible solutions.