Dave Plowman (News) wrote:

Masking tape can work - starting out with a small drill. Or if you have
one of those old tile scorers - the sort you use before snapping them.
Just used to break the glaze.

Since the surface of the tile immediately around the fixing holes will be
covered by the base of the mounting brackets for the grab rail, I have
wondered about trying to scratch off the glaze where the holes have to go in
order to provide better grip for the drill bit. Something to try on my test
tile.