Yes, but business casual and casual fridays are two different things.
I thought to describe as something one would wear to church - but that means different things to different people.
Yes, there will likely be people wearing jeans in the MDR (there always are). If they are darker in colour, and you dress them up a bit, and you absolutely must wear jeans then go for it. i don't think anyone will ban you from the MDR. However, if you want to go with the flow and blend in, then I'd go with the following as suggestions for the MDR for non-formal nights:
-pants (non-jean) or skirt with dressy top (not something you'd wear to the beach!).
I have a studded/beaded t-shirt that I have worn to the MDR but other than that, I would not recommend tees for the MDR either.
If you want to fit in (and I suspect that you do) .... I'd wear jeans on the first night and skip them for the rest of the nights.