Like the look of that, someone has obviously made one, trick is going to be finding it, could be a one off looking at the rest of the back end, or lack there of.
When I was looking at the pics posted by Rigsy, I had a bit of an inspiration. What about a flip down splash gaurd? A bit like those flip up registration plates...
So you would have a plate hidden behind the reg plate when it's dry and it flips into place when it's wet. WOuld need a bit of home engineering though.
I agree that the Ducati-esque splash gaurd would be a better proposition, but where to get one.... Would have to DIY I guess.