Here are several abstracts about using LED light therapy for this problem.
http://www.healinglightseminars.com/laser-research-library/dry-mouth-sjogrens-syndrome-case-study/
The articles may refer to LED light therapy as low level laser therapy.
This is available at physical therapy departments, and other medical professionals use it too.
You can purchase LED light therapy units yourself, but they are usually not as strong as the professional models.
An example is the SOTA LightWorks.
http://www.sota.com/lightworks.html
Hi and welcome to the medhelp forum. Dry mouth usually results from salivary glands damage during radiation therapy. This may interfere with eating and drinking with difficulty in acids being neutralized. You could use a straw to drink liquids if it is difficult otherwise. Few drops of glycerine added to water help in easing the discomfort. You must avoid sugars, alcohol, smoking and caffeinated drinks. Usually dry mouth improves after the treatment but if it persists, you must consult your physician immediately. Wishing you all the best.