I have not made bagpipes before, but I may be incorporating one into an upcoming Scottish groom's cake. Although I like making everything edible, I'm leaning toward covering pvc pipes with gumpaste. I think that is the best way to keep them uniform. I'm also doing the tartan icing. I'm going to print the plaid design on edible paper so it will be exact.
Don't know if this helps, but since I'm making something similar, I thought I'd offer up my thoughts!