I think this is a fantastic question.
As far as the Parkinson's application, it's definitely within the realm of possible. Taken form a recent review:
Preclinical research in animal models of several movement disorders
have shown variable evidence for symptomatic benefits but more
consistently suggest potential neuroprotective effects in several
animal models of Parkinson’s (PD) and Huntington’s disease (HD).
Clinical observations and clinical trials of cannabinoid-based
therapies suggests a possible benefit of cannabinoids for tics and
probably no benefit for tremor in multiple sclerosis or dyskinesias or
motor symptoms in PD.
Kluger et al. in Movement Disorder, 2015 (The Therapeutic Potential of Cannabinoids for Movement
As far as every-day hand steadiness, I haven't been able to find any good sources yet. I'll keep reading, and update this if I find anything interesting.