My Post does not have a MA and I do not know of any provision in the Nat'l By-Laws & Ritual that allows a Post Commander to Appoint a MA member to Chair a Committee of the Post.
A SOP does not override By-Laws be they Post, Dist, Dept or Nat'l. In my Post, when the Cpt of the HG decides to step down, he selects his own replacement being a Co-Cpt or Lt of the team and the Post Cmdr is agreeable to ppoint the new Cpt as HG Chairman because the Team supports the transition.
As I mentioned before, a Chairman is responsible for running and selecting his committee members. A way around this MA problem/dilemma is for the Post Cmdr's appointed HG Chairman/Captain to appoint a Co-Captain or LT, to run the team/committee in his absence, and also appoint a HG Adjutant (if/when HG Capt is absent) to make HG Reports at Post Meetings and make motions for expenses, supplies, etc.
This evening I did submit to VFW Nat'; FAQ the Question: Can a Post Cmdr appoint a MA member to chair a Post Program such as the Post Honor Guard?