In case anyone else tries to find light weight ways to ship beer, Kegkeg supplies cardboard kegs to roughly ~20 Belgian breweries. However, they're less popular because they're more expensive than metal kegs. The 20 liter keykegs are more rare than the 30 liter ones, and so a min order of ~100+ kegs is often required.
It turns out it doesn't matter in my case.. I discovered the country I planned to import the beer to has a 1 liter limit :annoyed: Anything over 1 liter of "alcoholic beverage" is automatically deemed to be for resale.