What we did is set up a percentage of approvals. We have 12 CAB members and 51% need to approve. That means if one or two people are out of office/ in meetings/travelling, it really doesn't matter. If we have 7 people out of office at the same time, we have bigger issues. That being said, if someone delegates their responsibilities to someone else while out of office, that delegate can contact the change management person(team) and we can approve for that person on the back end as Change Manager role. If this is too frequent, you can also just go into the change advisory board group and unlink the person and link the delegate. The danger to this option is that pending approvals won't see the changes. And additionally, when the person comes back, all of those pending approvals would still be with the delegate.
John is out of office and Steve is his backup. Steve contacts me and says he approves for John. I go into the change and flip from "pending" to "approved" and enter notes in the change that I did this and why.
However if you do the link/unlink option, the change would still be John's approval and not Steve's. You'd have to change all of the approvals to Steve manually. This can be done via approval workspace, sort by name and just flip all of them to the other person.