Yes, but you have to arrange this before the enclosure is made. You can't drill the glass after it's fabricated.

I was given the option of having the towel bar connect to the door handle like this.

writersblock is 100% correct it must be done during fabrication as once the glass is tempered all bets are off as the glass cannot be altered (will shatter into a million little pieces).

Attaching stuff after the glass is installed requires suction cups, or over the top hang down hooks, or Command hooks for wet places. I've heard of epoxy being used as a last resort.

If you haven't yet ordered your glass, you can put them anywhere you'd like, so you need to decide that when you first order the glass, before it gets tempered as Writersblock said. It would be a good plan to have the towel bar, or robe hook in hand when the glass guy measures, so he'll know where to place the holes. You can a horizontal door handle that doubles as a towel bar.

-Babka

thanks for all the advice.