Under User Pane enable "Hide User Picture", then under User Pane => Text Settings set the "Horizontal Text Alignment" to Left, then under User Pane => Margins adjust the "Minimum X value for text" X being the number of pixels to shift the User Name to the right from the left context margin.
This works for titlebars with/without system icon. I'm thinking it should work here as well. Could be wrong, but worth a shot
