Normally the hospitals have people to do it if the patient is in the hospital. My work colleague father had to have an emergency op in Sukhumvit hospital and after his 15 days visa was done the hospital went along to immigration and sorted the extension.
all went smooth apart from the traffic. I found the officer on duty very friendly and helpful. Gave me a piece of paper showing how to do it online next time. I've heard it rarely works though? Any tips because it did basically take up 3+ hrs of my day to drive out and back