Is a 6 in. hole in a band joist okay?
You have not mentioned the size of the joist. If it is 2 x 10 or larger I don't see a problem (heck, 2 x 8 might even be fine). The band joist (I call it the rim joist) is primarily for providing an attachment point for the floor joists and for holding the floor joists plumb and true. The load of the wall and structure above is distributed across the bottom plate and all the floor joists, so I do not think there is a structural concern here.