In my experience, the most important thing to packing in trails and making them rideable (for fat bikes anyway) is lot and lots of traffic. It doesn't matter what kind of traffic, bike, ski's, snowshoes, just regular hikers/runners, you just need traffic to mush the snow together and down or push it of the trail.
Second, I'd like to reiterate that singletrack trails are not the only place to ride a fat bike. Frozen lakes, streams and rivers work (for the flowing water its best to know the river/stream well so you know where weak spots in the ice might be).
And then there are snowmobile trails. Though I don't think bikes are technically allowed on the snowmobile trail network that crosses private land and such. However, they can't kick us of the Badger Trail, the Glacial Drumlin Trail and the Military Ridge Trail. These trails are already really well groomed and ready to ride. Always yield the way to the snowmobiles, you can hear them coming a mile away so its not hard to do. I always also assume they are all completely loaded, so if I can I will yield the way behind a tree or something. To be honest, I often ride on the non-state-trail portions of the snowmobile network, again just show proper respect to the motor heads and I have never had a problem, just friendly waves or indifference.