As the Outreach Coordinator at the West Michigan CISMA I've been working on getting in touch with landscape professionals to train them on invasive species. Here is some information on my experience that might help you!
I've noticed that it is particularly difficult to get a response on surveys during the busy season. If you want a good response I would try a more involved survey method than just sending something electronic. I put together a survey that I've sent out to individual companies, Associations, and will soon be handing out at a field day.
As for 'certifying' contractors, a good place to start is to look at MDARD's list of businesses with a Pesticide Application License (http://www.michigan.gov/mdard/0,4610,7-125-1569_2459-11993--,00.html). It has companies based on county and you can see what categories they have so you can easily narrow all the businesses in your area down to just those with the legal ability to treat. You can then survey those companies on their knowledge of Japanese Knotweed.
I'm also currently working on a "Stamp of Recognition" program that our CISMA will give out to companies committed to using less, or no, invasive species in their landscape designs. You could do something like a seal of approval of contractors that you find that ARE capable of treating JK well.
I hope that something in the above paragraph will at least get you started!
Good Luck, Aubri