Remodeling an old mechanic's garage into a conference space
Many red flags. For one, that's not the right way to insulate a ceiling: if you want to use batts, you need to put rigid foam above the roof decking or leave ventilation channels between the roof decking and the insulation. For more info see http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling

Furthermore, the fire code is nothing you want to be trying to skirt or do the bare minimum with. Fires are serious business. Do it right. An alternative to drywall is something called intumescent paint but I'm not sure that would work well in this application because the paper facers of the batts are too flexible and brittle to ensure continuous coverage for the life of the building.

It sounds like you need professional help, especially if you are planning to turn this into a commercial/institutional space. There are liability issues associated with potentially unsafe DIY work in buildings that will serve the public. Spend more money up front and have a better, safer, more nicely-finished building.