My original question was based on a scenario where an existing member seeks to appoint another member as an “acting member”. In other words, a quasi proxy arrangement. As far as I am aware there is no scope for proxy arrangements for SC’s.
You’re right that there is no mechanism for proxy memberships of committees. What tends to happen is that the absent owner nominates someone and the committee decides whether or not to allow them.
in short, an existing member can’t “appoint” a proxy but they can nominate someone to represent them, subject to the committee’s approval.