Thanks Palimpsest for your explanation, sorry to take so long to get back to reply. Your first two paragraphs make sense, and others (builder, bathstore personnel) have agreed it's not a good idea to drill other things into the mirror. We were going to try to choose an alternate lighting fixture for this guest bathroom, but they had already glued the backplate on to the mirror, so we'll live with this one. But at a later date, if we ever did want to change, will that be "impossible" without risking breaking the mirror to remove the backplate? By your last paragraph, do you mean that if we put up a train rack (or whatever... say smaller towel rack or toiletry holder) that required drilling, that we just have to know where the pipes/whatnot are behind the wall before accidentally drilling into them? These tiles, being "very hard", apparently went through a lot of drill bits for whatever they did need to drill for. And presumably a train/towel rack should be mounted to stud underneath too, for extra stability? Actually, at some point I think we'll want to have a grab bar installed... so same points would apply, with extra of figuring out best angle/height to suit most users (being guest bathroom with potentially some heavier and/or older visitors) and ideally also esthetics. Eventually we'll aim to have grab bar(s) for master ensuite too, so whatever we learn will be useful there too.

Drilling into grout is extremely easy, drilling into marble is fairly easy, drilling into porcelain is a bit more difficult. Depending on what I'm drilling, I'll use a carbide bit or a diamond bit. Drill the holes, squirt in a blot of caulk, insert the anchor (if needed), and screw in whatever you're mounting. The biggest risk in cracking a tile comes from tiles that are not fully backed with thinset, and you can add in excessive force used by the person on the drill (trying to push the bit into the tile instead of letting the abrasives do the cutting), and along with all that, thermal shock. All that said...I've never cracked a tile when drilling. Not sure what a handyman would charge for it, the caution there would be the handyman not having the best bits for the material being drilled through. If your builder is on the hook, be there to assist with the layout if that'll make you feel better. He'd be liable for the whole shebang, in terms of R&Ring a tile if it were to crack. Have a level to make sure the bars are level, have a sharpie marker to mark the proposed screw locations on the tile. If you brought in a 3rd party to do the drilling and they cracked a tile, it's possible a shady mechanic might blame poor tile installation as to why the tile cracked, taking himself off the hook for any repairs.

Maybe consider using a curtain rail support for the rod?

here is my link to my reveal several years ago. http://ths.gardenweb.com/discussions/3333818/my-west-bathroom-reveal-from-project-in-2013-to-2014?n=15 apparently the link in the tread to show my pics doesn't work any longer :/ I'll see what I can do to that. here is the floor with the shower pan. The Hydroban wall moisture barrier is not showing any longer, its under the shower tiles: Read the reveal text that I linked above. It will tell all about everything I used in this bath remodel: Floor with unfinished vanity, across from the shower. There are 2 doors into this room, the one you see here and on the other side of the room facing this door. Its my bathroom hallway, lol: Have to show my laundry center, across from the toilet. My beloved Miele washer and dryer: