Does tremor frequency generally increase as Parkinson's disease progresses?
Frequency of tremor typically remains constant for the person. Levadopa (sinemet) does not change the frequency.  

Typically the tremor frequency will be in the range from 3Hz to 7Hz.  There is a common phrase when dealing with PWP (Person With Parkinson's Disease). If you have seen one PWP, you have seen one PWP.

Currently I am part of a study by the Micheal J Fox Foundation using Pebble Watches and Smart phones to monitor PD.   

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1014720/
Hughes AJ, Daniel SE, Kilford L, Lees AJ (1992) Accuracy of clinical diagnosis of Parkinson’s disease: a clinico-pathological study of 100 cases. J Neurol Neurosurg Psychiatry 55: 181-184.

https://blog.getpebble.com/2014/08/25/mjff-intel-parkinsons/