It is currently impossible to set default relations across project types; for example, in Minecraft's case, mods can't be in resource pack default relations, and resource packs can't be in data pack default relations and so on.

For example, I own a Datapack called Custom Music Discs that requires a Resource Pack with the same name, but I can't set it through the Default Relations because it accepts only projects from the same category, so Default Relations for datapacks can only be other datapacks.