I suspect, with a few modifacations, the black tank might be easily modified to take the sink water. The shower water would likely be another story, though much more possible, than, say, retrofitting the kitchen sink to flow uphill to the black tank....
The problem is, the Ollie has a "marine bath." Like most marine baths (even the large head in our sailboat, with a separate shower compartment), there's a curb to create drainage, and a stepup to the toilet to allow some space for the black tank. And, the inlet to the black tank is directly beneath the toilet. Who wants another nonfactory inlet, subject to leaking?
In many of the big stickbuilt rvs, (actually, all the ones I've owned and rented over the years), the toilet is floor mounted, and the shower is elevated. The shower water could easily be diverted to the black, and maybe is, in a few cases.
The composting Nature's Head is, from my conversations with the reps, much more popular so far with boats than rvs. Why? Finding a legal pumpout station is much more difficult with a sailboat/houseboat than finding a dump station with an rv, in my humble opinion. We never have a tough time finding a dump station with the Ollie. Special trip (raise the anchor, go to a marina, etc.) with the boat is another story...
It's true, most of us who have purchased Olivers are likely to be lifetime owners. I suspect we are. But who knows? We're in for eight years so far... but?
If our trailer were not plumbed for a typically desired toilet, and we wanted a Nature's Head instead, we'd be forced to sell to a really narrow market in today's world if we decided to sell. Not ideal.
I think Oliver is wise right now to "duplicate" systems with extra plumbing and a black tank, inoperable, or perhaps linked only to the sink. At least you'll never be stuck with a really narrow resale market if you order the composting toilet.
By the way, when we looked at the Nature's Head, I think it was about $950, install it yourself. If that's still true today, I think Oliver is offering a great price at $800. Sure, you don't get the usual Thetford. But you get everything else, and a complete, pro installation with your new Nature's Head.
My 2 cents, and probably only worth that much.
Sherry