In constucts such as "@(ItemGroupName->'%FullPath')", the "FullPath" should be completable. Use the "well-known metadata" and metadata names defined on the item in this and included files for the candidates.
Note 1: "ItemGroupName" should be completable as any normal item name.
Note 2: Sometimes it's acceptible to write just "%(FullPath)" (if the "ItemGroupName" context is known) or "%(ItemGroupName.FullPath)", these should also be supported.