Hi dr jonny,
I haven't yet found any proper evidence of causality between cortisol de-regulation and parkinson. So far, I only have a fair amount of what you might call "circumstancial evidence" that cortisol is important, as follows:
1. A few studies (with proper control groups) have shown that PwP have low levels of cortisol. See, for example this one: http://www.ncbi.nlm.nih.gov/pubmed/1851513
2. My own lab results show that I have low levels of free cortisol (apparently because I have too high levels of transcortin: most of our cortisol seems to bind with transcortin, so when you have too much transcortin, too little cortisol remains free to do the stuff it normally is supposed to do; my levels of total cortisol seem to be normal - it's only the "free" cortisol levels that are low). I also have a few lab tests that suggest inflammation, and I obviously have PD too.
3. Cortisol seems to be widely believed to play a key role in the way our bodies' response to stress, inflammation and conversion of sugars and fat into energy (just check wikipedia or google cortisol). I don't fully understand yet how this works nor how exactly it is linked to PD, but I find it suspicious that stress, inflammation, and energy (tiredness) have also been clearly associated with PD. There's also a bunch of other similar "loose" associations that I find interesting - if you google what natural things you should do to get a normal level of cortisol, you find many of the same things that people often mention that sometimes work to alleviate the symptoms of PD (moderate sports; cafeine, etc, etc).
4. There are some claims (which I don't know how true they are) that giving PwP with cortisone helps with their PD symptoms. See, for example: http://www.prweb.com/releases/2012/9/prweb9954106.htm. Note however, that I'm personally suspicious of this approach to increase cortisol levels, as taking cortisone apparently has some important drawbacks (like you get "hoocked" for life, as your body stops producing the stuff naturally) - so I'm not keen on going down this road - at least not with the level of info I have today on cortisol.
5. This article (http://www.ncbi.nlm.nih.gov/pubmed/17414942) concludes that taking L-dopa to treat our PD symptoms also further de-regulates cortisol... which means that if you believe the hypothesis that cortisol de-regulations is a cause of PD... then it makes it even more important to find a way to manage your cortisol levels when you're taking l-dopa (otherwise you might just be treating the symptoms of PD but actually making things worse).
PS. I get that science progresses slowly. It's just that I don't have that much time :-)