Along the lines of being for a shift cable pulley, could it be the "pulley" itself? Ie, the cable running under the cylinder like it was a pulley. If so, the "pulley" would just be a washerlike cylinder curved on the left side for the chainstay. Hole would not run through and the inside wold be painted. The location looks right for a clean lead form a downtube shifter.

Edit: the oilport can be seen on the BB shell.

Ben