Hmm, yes, pinholes would have quite serious implications for the structural integrity of the frame. Is there any way I can check for this without blasting the paint off the frame? I assume not.
Take the post out....which sounds like it may be simpler to say than do....gaffa tape a maglite or such like facing down the seatpost. Turn it on in a darkened room. Helps to use a mirror to spot any pin holes of light through the frame.