Dear Albert,
The (potentially curved) reference axis of BeamDyn defined by the key points is not tied to a specific location within the chord of the airfoil. Indeed, the beam model of BeamDyn does not care where the airfoil leading edge, trailing edge, or other aerodynamic surfaces are. Instead the BeamDyn reference axis identifies the origin and orientation of the cross-sectional 6x6 stiffness and mass matrices. It should not really matter where in the cross section the 6x6 stiffness and mass matrices are defined relative to, as long as the reference axis is defined consistently and closely follows the natural geometry of the blade.
When coupled to FAST, the aerodynamic geometry and discretization is defined in AeroDyn, independent from the geometry and discretization of BeamDyn.
Best regards,