In one of his books, within a self healing practice there is this under Guru Mantra:
OM AH GURU VAJRADHARA SUMATI MUNI SHASANE KARMA UTA WARDANYE SHRI BHADRA WARSAMANYA SARVA SIDDHI HUNG HUNG
I don't know if this is a generic guru mantra or is his name mantra, it is writen under a picture of him in his Mahasiddha pose.