The easy solution that I found was to use a Text Widget dropped into any suitable area supported by the Theme.
First, add a text widget to your site by opening the Dashboard then picking Widgets from the Appearance menu.
The Text widget allows a block of titled arbitrary text to be included in every page displayed where the theme includes the area holding the widget. It is intended for things like site-wide disclaimers and notices.
The widget has two text entry boxes, either of which may be blank. To get a suitable BY-NC-SA logo inserted, use the CC license wizard to create a suitable chunk of HTML, then copy and paste it into the body of the Text widget.
When you paste the HTML into the Text widget, some of the information will vanish. I believe this is because WP doesn't support the RDFa extensions used by the wizard to record all the info about the rights holder.