I have a couple of suggestions.
A heated humidifer added in line with the bipap would probably help the most. The heated humidifer helps dry mouth, stuffy, congested, or runny nose, dry, burning, itching, or bleeding nose, and a heated humidifier will either reduce or eliminate these symptoms. Allergies cover most of these symptoms.
You might also ask your physician for a prescription for an allergy medication and or advice on any other possible solutions.