Pontius, Its a long time since I crawled down a Lanc fuselage, so I can't comment on that hatch. Was it not the astrodome there? From discussion with crew members, it seems the front (Bomb Aimer) hatch and rear door, were the favoured routes. That main spar not helping the W/op or Nav. The pilot had a hatch I believe, but I don't think it was that practical.
Plenty of stories of Lancasters breaking up and the pilot surviving as he wore a seat type chute.