I'm sure you'd be fine in capris. We saw plenty of capris, even shorts, in the dining room on regular nights of our last cruise, but that was a shorter one too. I have heard before that on longer cruises, the dress on non-formal nights in the dining room seems to be a bit fancier than on shorter trips- for example, business casual.
Don't know if I'm correct in this, but either way I'm going to assume that you'll be fine in capris outside of formal nights.
Majesty OTS, 4/07
Mariner OTS, 4/08
Adventure OTS, 4/09