If you have a 20" wheel in a supposedly 26" fork there should be loads of space for you to cobble something together, maybe you can run p-clamps around the fork legs, and bolt your very own mudguard bridge to that?

Or a length of those galvanized perforated steel strips used to hang acoustic ceilings and all kinds of pipes and ducts from perhaps. Some shrink-wrap tubing as a sleeve to protect the paint of the fork and then you're good to go.