Its sounds like you're coming from the cider side of it, but I can't imagine its any different than beer. I've forced carbed edwort's apfelwein before and quite enjoyed it. The pressure you use will be determined by the temp of your freezer and how many volumes of CO2 you are aiming for. Here's a chart to get you started
. I did the apfelwein at 2.5 since that's what I've got my beer at and thought it worked well, but if you wanted to go higher than that you could increase from there, or even step it up along the way until it got to the point that you wanted it. Should point out though that at least for beer higher pressures require longer lines to prevent foaming. Since this shouldn't foam, not sure if that would be necessary or not, but short lines will likely knock some of the CO2 out of suspension when it exits into the glass, so you might reach a point of diminishing returns.
As far as the additives and backsweetening, here's one part where this should be easier. In theory, you're going to keep your keg at such low temps that there shouldn't be too much worry about yeast activity. Further, since its in a keg the end result if there was activity wouldn't be bottle bombs but a slight overcarbonation. I haven't tested this so maybe someone with more experience has, but you shouldn't need to even stop the fermentation. Make sure you get your keg cooled down before any backsweetening and keep it cold, and I would think you should be safe. Again, might be best if you get that confirmed by someone that's done it, but that would be my thinking.