Both papaya and pineapple contain enzymes called bromelain(e)/papain which are reported to break down hair and you can even buy dried chunks from various pet emporiums and health food shops. However, there is debate as to whether it actually helps. Some people swear by it - and insist it works well. The Harcourt-Brown textbook of rabbit medicine suggests that she believes it is "nothing more than a sugary treat": there is also concern that in an animal with a sluggish gut, giving extra sugar will do more harm than good because it will both change the pH of the gut and sugar creates hormonal changes in rabbits which also slow down their guts. The enzymes are also destroyed by heat, so pasteurised/tinned products including fruit juices will not contain the active enzyme.
Personally I would avoid it and just ensure that the buns have access to plenty of clean, fresh water and leafy vegetables to keep them well hydrated, and use some kind of brush/glove to help remove as much as the loose fur as possible to stop the bun from ingesting too much.