I’ve never tried a drainpipe, but I see it’s been a while and no-one else has answered. I would think a 1 metre would be a bit long – I would be worried that a large hog might get stuck in that.
Not sure how much trouble you want to go to, but I had what is, maybe, a bit of a whacky idea. You know the entrances to some hog boxes, where they go in the door and are met with a wall in front of them so have to turn immediately left or right. Normally cats, and I imagine dogs find that a bit tricky. But maybe just sticking a piece of wood in front of the hole so that the dog was met by an obstacle but the hog could quite happily turn left or right? You could even put up more than one ‘baffle’ if it is a Houdini dog? But maybe, if you wanted to, you could make a sort of posh hog ‘lobby’ with something like that in it? Just an idea – never actually tried it!