Has anyone heard about some type of mouth guard that helps with cervical dystonia ?

No: Cervical dystonia is not a curable problem, but it is treatable. Oral medications, physical therapy, and carefully planned Botox injections are all helpful therapies. I'm not aware of an oral appliance, such as a mouth guard, being useful in its treatment. I would check with your treating neurologist, or considering contacting the mayo clinic or cleveland clinic if no answers found.

Cervical Dystonia: As a dentist in practice for over 40 years, I have never heard of a dentist fabricating any type of oral appliance that would help correct or manage cervical dystonia. You could run this by a dentist with extensive training and experience treating patients with tmd\tmj although the real experts in this filed are few and far between.