Attaching a towel hook to a glass shower door
You could use over door hooks, such as this, on the bathroom door.

Google suction cup hooks for hooks that attach to glass.

Supermagnet hooks are also available. You could attach a decorative metal plate to the bathroom door for them. Or use a supermagnet hook on one side of the glass door and an non-hook supermagent on the other side






EDIT:
Here is a link about attaching metal to glass.
What type of adhesive do I use to bond metal &amp; glass?

The accepted answer recommended epoxy.