Three seminars/conferences? In AUGUST!!?? Wow, don't they know August is vacation season?
If you are looking for "professional casual" vs "professional meeting", in an engineering environment , I'd say white jeans would definitely work. I think I'd avoid BF's, but straight legs or flares would be fine. I know I've worn mine to a pre-conference meeting and didn't feel too casual. You say your skirts are too "meetingy", is it worth purchasing a knit (thinking jersey) tube or pencil skirt? I live in skirts like this in the summer, as they are comfortable and have that casual summer vibe. Or a drapy t-shirt dress with a cardigan?
I have a big engineering conference in Sept every year, but I suspect it's a bit more formal - I usually dress meetingy (suit even) for the first day or two ... but the pre-meeting is pretty business casual to casual, and the evenings I wear fairly "normal" go out clothes. Jeans, dress for the banquet, that sort of thing.