As an employer you can have your employees do anything you want within the bounds of the law. You can have them scrub toilets or pick up dog shit if you want.
I see nothing wrong with asking employees to come up with marketing ideas or plans as part of their jobs as long as they are expecting them to do this while they are at work and on the clock and not when they are at home on their own time.
If it is a really small business as in one that does not have steady flow of customers the people working there likely have a lot of idle time. Nothing wrong with asking them to do something for the good of the business while they are waiting for customers to show up.
All this being said it is a lot easier to catch flies with honey. I have been managing people / running a business for about 15 years. By FAR the best way to get people to do anything is make it where there is something in it for them.
For example I was struggling one time to get production rates up so I told my employees that every day you meet this quota I will give you an extra $20 as a bonus. They started busting ass and met the quota. They even ran off some of the slower employees that couldn't keep up so they would be sure to get their extra $20 each day. It was a win-win all the way around in that they made more money, I made more money, we thinned out weaker employees, etc.
In the scenario you describe I would maybe make it where any new business from the employees ideas they get a cut of. It doesn't have to be huge to get the desired effect just something that makes them want to do it.