I'm not familiar with Michigan's no-smoking law as it applies to hotels. Is smoking forbidden only indoors or anywhere on the property? If it's only forbidden indoors, why don't you either:
1.) Look for motels, where the employee only has to step outside his or her room to smoke.
2.) Look for hotel rooms with balconies where the employee could smoke on his or her balcony.

If the law applies to smoking anywhere--indoor or out--on the hotel property, you'll have to look for a hotel/motel where the rooms are located close to the road. Google Streetview should help with that.

I appreciate that you're looking out for your workers, but frankly this is the direction that cities and states are moving. It's a problem you'll continue to encounter. I don't know what part of the state you're in, but fortunately you're in Michigan during warm-weather months so smoking outside shouldn't be too much of a burden. The smokers should consider themselves lucky that you aren't in the UP in February!

(Also, please keep in mind that any hotel that unlawfully allows your employees to smoke in rooms is choosing to expose its employees to second-hand smoke. They're also opening themselves up to lawsuits from their employees. With this in mind, I suspect you'll have a tough time finding a hotel to break the law on behalf of your employees.)