Dave - Perhaps the engine room, ship's laundry, or that weird place below the galley where they flatten out the cardboard boxes for recycling?? I think that casual dress would be accepted at any of those "venues", although the views from those venues might be a bit dreary.
As for the pool and jacuzzi, I don't recall ever seeing anyone in either of those wearing formal dress (except for that one rather dipso lady who tried to demonstrate that she could walk along the edge of the pool in her very, very high Prada heels).
All things considered, my vote absolutely goes to T-shirts and cargo shorts 24/7, just as I wear at home. However, I recognize that such a sentiment is not likely to ever be adopted by the dress-code nazis at Regent; so I would vote for full-time "elegant casual". We have loads of comfy tropical silk shirts (worn untucked, of course) which are not only elegant and casual, but also fit just as well on day 1 of the cruise as day 15.
Go Casual!!!!
Cheers, Fred