"Casual" is the worst word in use on invitations! Too vague.
I'm with Tobus--jacket or waistcoat, plain hose, etc. A sweater may be acceptable as well. This is what came to mind when I read your post:

Not a great pic, but a fair illustration of my intent.
I think it's safe enough to work without being over or under dressed. Maybe have a tie on hand and wait to see a few people walking in and put it on if they're wearing one, and leave it in the car if they aren't. I'm never fashionably late...I'm in the parking lot making sure I'm dressed right since people insist on terms such as "business casual" "semi-dress" and "jacket preferred"!
The grass is greener on the other side of the fence...and it's usually greenest right above the septic tank.
Allen
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