This could be something as simple as a well publicized video showing after work at a nearby location, or as ambitious as a rally outside the shop at shift change, built with the aid of other folks from your area's Mumia movement. One good way to go is to organize for a single day where supporters of Mumia can wear a button, a ribbon, or a T-shirt to create a presence and a stir around the issue. With ribbons, you can give them out to anyone who will take one. T-shirts are more expensive and will not be worn as widely, but are also much more visible. Overall, try to make the entry point for the activity easy.
People are much more likely to wear a free ribbon than a button which they have to pay a dollar for.
If you are sending postcards, have them prepared so people can just sign them, but leave room for them to write their own comments. Build your activity boldly. Leaflet at the entrance to your workplace or in the lunchroom or in the locker room. Remember you want people to get to know you, you want to become a pole for support for Mumia. As we stated at the outset, it is not the principal goal of this approach to get a union resolution passed.