Have a look at this previous thread:

http://www.physiobob.com/forum/neuro...-patients.html

The KNGF guideline is a good guideline and there is a updated UK version here which looks a bit more broadly including some issues with pain:

Guidelines for Physiotherapy Practice in Parkinson's Disease

However on the whole there ain't much out there for pain or rigidity treatment in PD. Muscle pain is common and is probably linked to rigidity - the cogwheel rigidity is often due to a combination of lead pipe rigidity and tremor that is masked by the rigidity anyway. On the hand it could it could be a more typical MSK presentation.

Have you fully examined your patients neck and arm pain pain? your comment on the cogwheel rigidity what is that based on?

What about palpation of the neck muscles? - they often seem to get trigger points. If the patient has TPs and you can reproduce the patient's pain that way may be a treatment option.

What about neck posture? could there be a movement impairment syndrome/muscle imbalance component? If so may be approaching the correction of the muscle imbalance may help.