If the Text Box has a Tab Stop set to "Right" (so it grows to the left), just take the X Position of the text box and subtract it's Bounding Box Width..
In this example I wanted a quad to be placed before the text object, so I did the above, and also subtracted Half the Bounding Box Width of the Quad (since the X position of the quad is in its center), and also subtracted a bit of extra space:
#XPression