We went to a birthing class last week which showed a lot of video, and the ladies in the gowns were pretty much naked. One lady that they showed with a hospital delivery had on a nursing bra (the sleeping kind that looks like a sports bra) and a loose cotton skirt, so they could still do the fetal monitor, the back was open for an epidural if needed, and she could change positions around the room, and after the birth she pulled down one side of the bra and the baby snuggled right in there. I might do that for actual birth.
DH mentioned it was weird they were all practically nekkid for the entire 4-10 hours of labor before the actual birth, so I think he'll be OK with me getting some sort of bottoms. The PJs he might have an issue with, but I don't have any dark colored ones and FJ has a good point!