Maybe the PO drilled it out, thinking he was drilling out the cap that the manufacturer fits over the idle-air screw?
How 'bout making a little tin cover that's just slightly larger than the OEM brass plug and super-gluing or even JBWelding the cover onto that little 'shelf' that you can see in the first picture. Something to cover the opening where the brass plug used to fit, but not plug up the internal passages?