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They probably don't have any, because of what dannat says - you wont get it to focus because you wont be able to rack the focuser in far enough. You would have to move the mirror up the tube to get it to work. I'll use my favourite analogy, and get you to imaging the focal length is a piece of string a certain set length; you can bend that string anywhere you like but you can not lengthen or shorten it. If you put a diagonal in a dob it means you would have to lengthen the string to get it to focus, but the laws of physics wont let you. You have to move something so you focus point is at the top of the diagonal. You could shift the focuser down, but it would be easier to move the primary mirror up.
The only time I've seen dobs with diagonals is in binoscopes (two newts set up side by side to act as a binocular) and in those cases the above problem I've described has been allowed for in the design.
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