Mug cakes in the oven?
A microwave has a very different mechanism of heating than an oven, and recipes for the two are not interchangeable. The results of making microwave-optimized cakes in the oven will be unpredictable. And this will be a much larger problem than having the right mugs. 

A better option would be to find a recipe for muffins or cupcakes that is intended to go in the oven, and bake it in the oven. Do test it in a small batch before giving it to 100+ guests, because not all recipes floating around are good - but still, it is more likely to produce good results than baking your microwave cakes in an oven.