The hole is placed directly underneath the air cond (ECS) packs and should therefore be part of them I guess. Looks like some kind of drain hole perhaps for mositure? I think you can even see it in the principle picture below, the right hand pack has a tube that goes to the skin to expel something.
Seems like the form of the exit has evolved, my photo is from early prototypes and it is a flush back-angled exit, your exits seems to be with an exit cavity.
7BOEING7 From United States of America, joined Oct 2012, 1034 posts, RR: 5 Reply 14, posted (4 months 4 weeks 1 day 22 hours ago) and read 2655 times:
Quoting bikerthai (Reply 9): Don't look like a drain hole for moisture. Moisture drains are relatively small and usually mounted flush with pressure valve.
Except on the 777 where they all have "little fins" that cover the drains. They are easy off/on for drain replacement. This was an FAA requirement to prevent any fluid (fuel) getting back into the airplane "bilge" during a negative pressure condition.