... When far offshore, no sane person uses the toilet anyway so
it's a day sailing issue only.

That's a pretty ambitious statment. When the deck is being swept by
waves and it is dark, rainy and windy I think it is wise to use the
heads below rather than hanging my ass over the rail or trying to cart
a bucket of ofal to the lee side. Still, I spend a great deal of time
offshore in small boats so I suppose my sanity is open to question.

More to the point, I agree that the porta-potty is the easiest,
cheapest and likely best solution. I did the same thing to a CAL 36
that I owend years ago and it worked well for camping. If I were going
to live with the thing I think it might be worth checking to see if you
can go with a type III MSD. You'll have to check out the power, space
and legal problems before you even get to the cost... If you are up
for a bit of mad scientist action you might look into modifying a
composting tiolet to meet your needs. My current boat has the holding
tank in a deck locker so there are no through deck fittings...

-- Tom.