i think the normal glues may fail to adhere to the fat content, PatB needs to be asked, he is a strong believer in the spray shellac, maybe you can just spray the shellac over the crown area of the shaft, then glue on fletching.
but with that kind of finish, sounds like those arrows would be an awesome fit for a eastern 2 fletch
look into that verdigris finish- old english. you could do that, on the crown, and then finish with the fat on the rest of the shaft-
you can do that verdigris finish without the green verdigris- and it will dry kind of clear- i believe- never used it but it looks amazing.
i guess you could glue your fletching on first then seal with the fat mix, but i dont think you would be able to replace a fletch at a later stage.
there may be some other natural sealants that may work as well, that are very primitive, like a natural varnish made from pitch and turpentine- i dont know the recipe- but i think i have read about Pat making some.