I had a pair of fairy wrasses which loved jumping into the overflow of their tank. To get them out I usually just let them find their own way out, but to "encourage" them to do so, I would turn off the sump line (i.e., I just attached a valve and closed it), then flooded the overflow so the water level in there matched the level in the tank, then I left them alone for half an hour or so. Usually they'd just swim back into the tank within a few minutes anyhow, but if they wouldn't still after that, I'd just throw some flake in and they were pigs for food so that was usually enough. I don't think I ever managed to net them out. I know one time when the the female was caught in there, I went to go get the net because she wasn't coming out on her own, but as soon as the saw the net coming she jumped out (and into the tank).
They were quite prolific jumpers though and that sort of leads into why I don't have them anymore. One night when I was away my fish sitter forgot to close the canopy after feeding them, and they both jumped that night.
My 6-line has never shown the same tendency to want to jump so it may just be because of this jumping behaviour that they were so easy to remove from an overflow. Thought I'd mention this technique anyhow, might be worth a try. If that doesn't work then I think Rory's suggestion of removing the internal plumbing is the way to go. At least you'll be able to get the net in there a little easier.