University Project - Common Mode feedback in EEG systems
No --  sounds like a good idea, but separating EEG and EMG is much easier said than done.  How do you intend to get  "pure"  EMG to use as your common Mode signal, especially if it shares a freq spectrum with EEG?

The real way to separate emg and eeg is filtering.  EEG is lower frequency than EMG.  What isn't low frequency, though, is movement artifact.  If the EMG activity is caused by movement, and the movement also causes motion in the electrode-skin interface, this can cause trouble.