In any county which has implemented the provisions of Article 15.6 (commencing with Section 31855), any member who is thereafter retired for disability shall receive a supplemental disability retirement allowance in the sum of three hundred dollars ($300) per month in addition to any other benefits due under this chapter, provided the member’s disability is such that the member is incapable of gainful employment. The board may adopt regulations, including a requirement for periodic declarations of nonemployment, to administer this supplemental allowance.