Matte boxes serve a purpose, they are important beyond making you look more pro. They are also just part of a system, In most cases you need a rail system to mount them to. Then of course there are the filters.
But I cannot believe that people would use a DSLR camera on a shoot without discussing it with the client. If someone put off by a still camera my guess would be, they would be put off by a still camera with a matte box.
Like Darrell stated once the client saw the results they were sold, but they should be sold before the start of the shoot. Just show them examples and simple say, I’m going to use the camera I shoot this with. Just my 2 cents.