Hydration certainly is an important issue surrounding this concern but, JP is still spot on with his statement that cotton mouth is a fight or flight response.
Possibly in this case, while no perceived threat truly exists, the body can still respond to it. For instance, even in paraplegics, a response will exist even though the nervous system connection has been severed. Just because they don't "feel the pain", the body does and will respond to it accordingly.
My recommendation would be to come up with a pre-match routine that can help stave off this response. It can be something very simple--ex. re-tying your shoes prior to walking out onto the mat every time you step out there.
Many successful athletes over the years used to do some strange things that were constant throughout their careers. Ex. Nomar Garciaparra's pre-pitch routine. Shayne Graham, place kicker for the Bengals, will walk down by the goal post and look for trash on the ground to test the wind. He knows that the flags indicate wind direction but, his little stroll before it is his time to shine is his way of getting his mind ready to complete his task.
Thanks for the time as always. I hope this helps.
Will Haskell
Athletic Development Specialist
siriusperformance@yahoo.com
willhaskell.blogspot.com