I do not know if it is possible to insert [ ]. However you can achieve the desired effect with parentheses and Graphite font effects.
Edit Footnote Anchor (character style):
• Font tab▸Font: Linux Libertine G:circ=2
• Borders tab set Left & Right Borders OnlyColor: WhiteWidth: Increase the value as necessary to obtain the desired spacing (e.g. 0.10 pt in FootNote.odt).