I consulted this wonderful thread before I had an overnight transfer in ZRH, and will share my experience back in hopes it can benefit others in the future!
I can report that there is definitely a shower room available for customers using the Swiss Business Lounge in Transfer Area D - the receptionist said it was a relatively new addition. I believe there is only one room - I was among the first in the lounge that morning, so did not have to wait. Others may want to keept this in mind. For the shower, you get a key at reception just as at the Swiss Lounge in Transfer Area A in exchange for your boarding pass, and the receptionist tells you to look for a certain door in the corridor with a specific number next to it; otherwise there's no indication it's a shower room. It's not the room with the icon of the shower on it (I think that one's for the Transit Hotel / Dayrooms?).
And I'm happy to say that this shower room exceeded my experience with the Transfer Area A lounge showers (at least the Business side - I've never been on the Senator side). The reasons I like this one better were:
1. There's a little shelf to put your bags, slightly lower than counter height. So they're not on the floor or hanging from a hook.
2. There's a rain shower head! And the water flow was excellent.
3. There's a clothes/towel rack in front of a heater.
4. Most importantly, instead of the wall-mounted dispensers of combo hair/body cleanser, there's this very nice Bulgari amenity bag! Besides this there were two other shaving kits, but I doubt that's standard practice. Is this the same one in the First Class Lounge?
Some more pictures from the lounge in my report
here.