I saw one a while back and thought they could be useful. Even just for cleaning kit while camping and such.
On a recent conservation effort in Crackpot, the backpack pesterside type sprayers were used with a fair amount of success. The don't have all that much pressure but more than a hand held bottle etc.
Some mud splatter in Crackpot was quite well stuck on so maybe having something like the Hydroshot would be good just to loosen the worst bits. I wonder how long the input pipe can be, I suspect not long.