MGB Design
How about making it a certification requirement that the MGB is dry-run-tested to identify the fail-point and to include in the basic architecture that this fail-point must not prevent the entry into safe autorotation.
Clearly lubrication of the upper casing mast bearing would be an issue but a 'bearing within a bearing' design with the outer (grease-packed) bearing pegged to the casing by shear pins. The inner (normal) bearing packs up under dry load and the shear pins break, leaving 'x' number of minutes of a grease-packed roller bearing to get you down.
G