Although some of the trails in Central Oregon are in the desert like Horse Ridge and Gray Butte, they are not always without snow, mud or crystallized frozen dirt in the winter. There are some BLM roads, east of Bend, that offer some more predictable winter mountain bike riding surfaces. In the summer, these roads are often way too dusty and soft, but in the winter vehicle use packs the dirt out pretty well. Just like the pumice single track trails, they get better with precipitation. The downside is that you will be sharing the road and environs with all types of off road vehicles, occasional abandoned appliances and paint ball warriors. The further from the access points near paved roads the less junk there is and the more you can focus on the interesting shapes of the big old juniper trees and the mountain views. Although not the ultimate single track experience it sure beats having to dress up in a professional road rider costume and put up with rednecks on the highway. ATVers are much more respectful of bikes when they are on their ATVs in the country than when they are in their trucks on the highway for some reason. Maybe they respect mountain bikers more than road bikers?
A number of irrigation ditches terminate northwest of Mayfield Pond and when the ditches are running it is swampy in that area. Monster truck owners frequent the huge year round mud puddles. Just east of the mud bogs it is high and dry with some excellent views of the cascades.
Signed routes for non-motorized vehicles do exist, but they either get so little use or they are so heavily used by horseback riders that they are too soft to enjoyably mountain bike. Maybe someday there will be enough bike traffic to keep some single track trails packed out in the winter. If you are training for a race or need a harder work out than the relatively flat roads, try riding some of the soft non-motorized trials like between BLM17 and BLM1 on the map.
If they are not shot to pieces or driven over, the roads have signs that declare them "designated routes" open to vehicle traffic but they do not have names or numbers. The BLM has no doubt decided that supplying irresistible named road sign targets would be a futile waste of taxpayer money so you are on your own with a map and a GPS unit. Speaking of ATVs and guns, this might not be the best place for a woman to go riding solo.
There are a lot of roads that are not on the map and new ones are being spontaneously added all the time by ATVs.
There is a possibility for a new type of extreme sport biathlon here though and that would be riding and shooting at appliances at the same time. The extreme part would depend on how drunk you and your friends got.
To get directions and a map to where the dirt roads start near the Pronghorn Club turnoff cut and paste Powell Butte Hwy @44.129610, -121.161840 into Google Maps as a destination. Enter wherever you are as a starting point. There are many other places to start from but this one is quick and easy.