maybe this will work for you:

1. Insert or draw the graphic in Data View
2. Drawing toolbar: covert graphics to features (delete graphics after conversion)
3. Feature is added to TOC; then set transparency % in layer properties

maybe this will work for you:

1. Insert or draw the graphic in Data View
2. Drawing toolbar: covert graphics to features (delete graphics after conversion)
3. Feature is added to TOC; then set transparency % in layer properties

please note: In order for the text box to work feature with transparency I had to drawing toolbar > New Annotation Group > Activate Annotation Group first.