The only cakes I have seen done like this are half normal wedding cake other half having tire treads and mud look to one side. But they still had a grooms cake. Can you convince her of that?
Maybe to make less little things the grooms cake could be the foot ball field with a football going towards the wedding cake with dirt splashed on the one side. I think this is still hideous but to me seems less cluttered.
Here is a pic of the cake I saw. So you could do the same idea only instead of a tire it could be a football.
JMHO as long as a cake is executed well ie pristine corners,figures look real, I never judge the decorator. I always know its not the decorators wedding its the brides and I know she chose the cake. I do think a more simple idea like the one above would also get you more orders of the same cake. You could also pass it off as unique because I haven't seen another cake like this.