Well, it should work if you define an angle, then call
ent_bonerotate for the "neck" bone when you want it to rotate:

VECTOR neck_angle;

.....

ent_bonerotate(my,"neck",neck_angle); //rotate it
neck_angle.x += 20*time_step; // rotate pan
neck_angle.y += 5*time_step; // rotate tilt

......

Play with the values in the lines where the vector gets modified; call the whole thing in a loop
Should work;