Bob,
I would never make a side agreement with a board member to allow any form of anti-dilution, which is what you are asking about. Board members, like the founders, are diluted with each round… What you can do is: upon completion of a two or three year vesting period of continued good service, consider a new grant of an option for a small amount to bring the board member back to the original percentage of the first grant, but vesting for another 2-3 years of service. Consider this: Granting 1% for 2 years of service each two years to three outside board members would equal 3% of equity every two years – and after ten years, your outside board members would own a claim upon 15% of the company. No outside investor or founder would accept that. With the method above, the three outside board members would at most still have access to 3% of the fully diluted equity for their continued board service…
-Dave

Thanks!

Bob Wallach
Bobwallach@comcast.net