IF there was a connection to the inside of the seat stay there would be no question about covering the hole. That could
easily be checked. From what I gather the openings are either 8Mx1.25 or 10Mx1.25 and are probably blind. My
inclination would be to find something to cover the opening to keep mud/debris out or dirt dauber nests, dirt daubers
like to build nests in small blind cavities.