Yes you can appoint acting members of a strata committee, and that would go to ensuring there is a quorum … you need to have it approved prior to any relevant meeting which the acting member may attend.
That had not occurred to me. At a previous meeting you can say have someone say “Jeff won’t be here next month so can Jenny take his place?” And that way the committee has approved.
But you can’t just have Jeff not show up and Jenny turn up in his place and expect to count towards the quorum.