Sounds like something else might be wrong.
If you have to do it try this. First cover drilling areas with masking tape, then lay out your holes onto the tape. Then draw 2 lines thru the hole center 90 degees to each other for reference in case you loose original mark. The tape will help protect if you miss when you drill. Now before drilling try a die-grinder with a small stone and try to get through the porcelain just a cat-hair biggger than the drill size. This may eat up the grinding stones, but better than flaking chips out of the surrounding porcelain.
Good luck, hope it works!!
Also if you are stamping letters into a tag, try the masking tape on the surface. Way more forgiving and no scratching, push the letter stamp into the masking tape. If it does not look right start over, when it looks right simply put the punch back into the mark and stamp.