I can't say I have seen one in use but did look into them a couple of years ago.  I thought it was an interesting idea.  I decided not to look into further when I saw that; sprayer and swather are needed at same time, boom whip looked nasty, there wasn't a simple guidance system available then (I tried a lightbar in a swather and found hard to use, I know there are better options now), water capacity was low, tires wre too wide for most spraying.

Sorry to be all negative but that is what I thought when I looked into it awhile ago.