That sounds like a brilliant solution -but don't you find it confuses already confused visitors? Getting used to a marine head can be daunting for noobs, never mind asking them to switch valves under the sink and having to turn on the tap? And after a few beers if they forget to switch it back, won't the head fill up with water when they (supposedly) wash up afterwards?
As for contamination, if bacteria could swim backwards up the bowl, against the current, out through the pump, along several metres of water line, wouldn't that be happening on our land-based homes? I'm not saying you're wrong, just that I don't know what I'm missing.