I do know that in the potteries industry they use a product (or used to anyway) to coat all their waste, its a flocculent of some type that is extremely slimy and coats whatever goes into a drain so that it cannot block the whole system up.
After saying all that I always clean up on site everyday, waste addy/grout into a bag and into their bin and waste water around the hedgerow. Never had a problem as of yet, if on site then we have a skip. The only way around it really is to get the customer to hire a skip on larger work or build it into the price, midi skips are about £80 in my parts. The last one I had though the operator insisted on no plasterboard whatsoever, anyone ever had the same?