I seem to recall a proxy of some kind being given to a Pub senator by, IIRC, Ted Stevens, who was ill. Don't remember what the vote was on, but it was on the Senate floor and not in committee.
Riddick's rules recite a requirement to vote from the desk. They further recite that a UC request, that a vote be recorded for a Senator not present, is not in order. The chair is specifically prohibited from entertaining a UC requrest t add the name of a Senator to a vote after the result of that vote has been announced.