The original Alberg 30 masthead sheaves were machined aluminum. I know of a replacement source that provides custom made Delrin sheaves.
Just wondering if anyone could offer a specific reason to oppose the use of Delrin in this application. The sheave is completely inside the mast, and the halyard is external. The Delrin replacement has a bronze bushing at the center hole.
(Replacing the sheave to move to all-fiber halyards...the original is machined for wire only on the sheave).
UV exposure would be my only concern, but as you said, the sheaves are within the mast. :D So, I say go for it. :D