I’m aware of several folks who have trained bipolar one-channel montages just below O1 and O2, at the top edge of the inion ridge, for balance and gait issues (certainly cerebellar issues) and claim to have gotten very good results.  They used a bipolar and inhibited 4-7 Hz while very mildly reinforcing 15-18 Hz activity. The EEG amplitudes are very small at this location in comparison with other places on the scalp, and the thresholds for reinforcement are adjusted accordingly so that the person is receiving ample reinforcement. They trained for 30 minutes.

This protocol has been around for many years.  It essentially places the electrodes on either side of the cerebellum, and it theoretically it makes no sense, since the cerebellum does not contain pyramidal neurons, which are the source of readable EEG signals  However, it has been reported anecdotally to have a positive effect in many reports I’ve seen. BTW, instead of asking a person to rate his own balance subjectively, it would be easy enough to put them on a Wii balance board and have the computer track it (and changes in it).