Are there any health risks involved in only consuming 20 grams of carbohydrates per day for an extended period of time?

No, as long as you are getting decent vitamin and mineral suppliments and have a good source of fiber. You'll need to drink plenty of water and if possible, make sure you minimize the red meat intake, focusing mainly on fish and fowl. Some people have found that the diet you describe is actually quite healthy and they feel much better.