Does moist heat really alleviate the symptoms of lock jaw and tmj?

Perhaps. Sometimes. If it make it feel better, it is helping. Make sure you are being managed by a board certifiec oral & maxillofacial surgeon.
Yes. Tmd or temporomandibular disease is a multi-focal complicated problem that causes inflammation in the jaw joint. Heat application will not cure the problem but may provide temporary relief.
Timing important. The moist heat may help with some facial pain and/or some tightness. But to provide the best therapy with the locking (closed lock is limited opening/ open lock you cannot close, ) you will probably need to see a tmj/tmd specialist. You do not want a dr. Or another professional manipulating the jaw without knowing what they are doing. The disc can tear and you may suffer many more problems.

Related Questions

Rly bad jaw/facial pain (Lockjaw/TMJ symptoms) following tooth extraction. Was prescribed Norco (hydrocodone and acetaminophen) but would an NSAID help me actually heal better?

See a TMJ specialist. After imaging and physical examination have excluded structural damage, management depends upon the particular type of TMJ disorder. Many posts-dental treatment TMJ patients frequently have accompanying symptoms of TMD and should be referred for appropriate management to a TMJ specialist. Read more...
Possibly. Remember Norco, really only masks the pain. Assuming you have no contraindications to take anti-inflammatories like ibuprofen or naprosyn, (naproxen) then it isn't unreasonable. Remember that medications like NSAIDs, can can irritate the food pipe/stomach so take with food. Avoid extra acetaminophen(Tylenol) above whats in Norco, as it's bad for your liver. If your pains controlled talk to your dentist. Read more...
Jaw pain. NSAIDs will help due to their anti-inflammatory action. Norco (hydrocodone and acetaminophen) is not an anti-inflammatory, but should provide relief of pain. You need to contact your dentist or the surgeon for a follow-up exam. Read more...
How long after? Not normal after tooth extraction beyond a few days. See an oral surgeon to determine if TMJ damaged. Read more...