I've done a simple "ginger ale" before just by grating a whole lot (no idea how much, but my arm hurt) into a gallon of boiling sugar-water. I cooled and pitched some yeast (shouldn't matter what type).
It fermented out really dry as you'd imagine, and was not too bad. Pretty one dimensional but definitely gingery. I think putting it in cider would be a great idea. You could boil in a small amount of sugar water and add the ginger (just to kill any nasties) to that before getting it onto the cider. I don't know if it matters whether you do in primary or secondary.
Try adding some lemon, lime, or any citrus really; and some type of chili pepper. Habanero is good, but hard to control. It is, as you say, pretty one dimensional with just ginger, no matter how much you put in, and I have put in a lot.
There is a thing called Ginger Beer Plant (GBP) that is like a kombucha bacteria/yeast SCOBY that will give it a tangy tartness. I am researching where to buy it from to get the real thing, as there are a lot of fakes out there. Even fakes who say it's the other guys who are selling fakes. It is what the original ginger beer/ale was made with, without even using ginger I think, but now most recipes add it.
Instead of wasting all that time grating, I suggest cutting it into "coins", then throwing it into the blender with minimal water. Steep/cook the slurry with some sugar to leach, sterilize, and preserve. This is a good way to have multiple sodas from one soda water tap. Just add a dose of you favorite syrup after pouring. No yeasty flavor, though. Although I guess you could make some "cask" club soda. Hah.
There is almost no way to have anything be sweet using table sugar and yeast without pasteurizing, chilling, or high ABV. I force carbonate my sodas, mostly out of laziness. This avoids several issues at the cost of some flavor, but preserves others. Most any natural flavor will be reduced if fermented. I plan to try a soda by partially ferming, then crashing/racking just for some natural carb and a bit of yeast esters.
For a ginger cider, I will probably just add it at keg time. All ferming seems to do to pre-ferm flavors is strip them. The ginger flavor fades fairly quickly in the keg as it is.
