I'm a bit late getting into this one...I've been offline for a while...
Connecting any toilet designed to use sea water to the onboard pressurized water system is a very bad idea and EVERY toilet mfr specifically warns against doing so because because it cannot be done without risk of e-coli contamination of the fresh water system, damage to the toilet or both.
You say you'd install a "check valve"...that's not enough...you'd need a vacuum breaker and redundant one way valves. However, that would only solve half the problem. The other half: manual toilets are designed to PULL flush water through the pump. Pressurized water PUSHES water through the pump, which will mess up seal, valve and gasket
alignment....creating more problems than it solves.
90% of the time flushing
with sea water will be just fine...it's only when the boat sits with sea water trapped in the system that odor
results. There is a very simple solution: tee your toilet intake line into the head sink drain line just ahead of the seacock. Last thing, after you've closed the seacocks, fill the sink with clean fresh water...flush the toilet. Because the seacock is closed, the toilet will pull the water out of the sink...rinsing the sea water out of the WHOLE system--intake line, pump, channel in the rim of the bowl AND the discharge. Or, if you want to flush with fresh all the time, just keep the sink drain thru-hull closed, either the seacock or a conveniently located shutoff valve...fill the sink with water each time to flush the toilet.
...although I've never understood why sailors worry so much about whether they'd be able to flush an electric
toilet in the event of a power failure. 'Cuz if you don't have even that much power, you prob'ly won't have lights, nav equipment
, communication, radar
or a whole BUNCH of stuff that makes the toilet the LAST thing on your mind...but you would still have a bucket! And anyone who's too dainty to use a bucket in those conditions...well...maybe I'd best rest my case here.