Plumb the gray and black water into the raw water pickup, and blast it out the exhaust back of the boat.
A good way to destroy an exhaust system, if not an engine! But it's not exactly a new idea.
There have been at least two attempts to market a system to use engine exhaust to incinerate toilet waste. The first was in the RV market in late '70s. It didn't last long 'cuz not only did the animal fats in sewage and the grease and oils in gray water destroy catalytic converters and clog up exhaust pipes, the exhaust STANK...making life miserable for vehicles who had to follow an RV up a hill.
There were two attempts in the marine market..the first was in the late '80s. A couple of Spaniards who quickly discovered (but not until after they'd spent buckets of money producing the systems and advertising em) that it only works in dry exhausts--but clogged 'em up and had the same odor issues RVs had....and besides, in the recreational boat market, at least 90% of boats have wet exhausts, which don't get hot enough to incinerate the waste. So it didn't last long.
A second system was tried in the '90s...even won some kind of "innovation" award. I bookmarked their site, but it seems to have disappeared...and a google search didn't turn up it or anything else either.
So I'm pretty sure that disposing of black and gray water via engine exhaust is an idea whose time hasn't come yet and prob'ly never will. But that won't stop someone else from trying it.
--Peggie