18RC and 18RG have the same bottom end (both cast cranks). The rods already have oil squirters in them for spraying oil in the direction of the gudgeon pin. Some of the oil should (in principle) land in the second hole in the top of the rod through to the gudgeon pin. This spray of oil would also cool the piston to some extent.
Becareful if drilling the block for oil squirters. You could easily make them too big and lower your oil pressure.
Edit - Or use proper oil squirters as MR DOHC just stated.