The basis for modern email, RFC-822, has this to say about Reply-To:

    4.4.3.  REPLY-TO / RESENT-REPLY-TO

        This field provides a general  mechanism  for  indicating  any
        mailbox(es)  to which responses are to be sent.  Three typical
        uses for this feature can  be  distinguished.   In  the  first
        case,  the  author(s) may not have regular machine-based mail-
        boxes and therefore wish(es) to indicate an alternate  machine
        address.   In  the  second case, an author may wish additional
        persons to be made aware of, or responsible for,  replies.   *A
        somewhat  different  use  may be of some help to "text message
        teleconferencing" groups equipped with automatic  distribution
        services:   include the address of that service in the "Reply-
        To" field of all messages  submitted  to  the  teleconference;
        then  participants  can  "reply"  to conference submissions to
        guarantee the correct distribution of any submission of  their
        own.*

(emphasis added to the last sentence) So, Reply-To munging isn't out of the realm of possibility. Also, the BNF for Reply-To does allow multiple email addresses to be specified. RFC-2822 and RFC-5322 both say the same thing about Reply-To:

    3.6.2  Originator Fields

        ... When the "Reply-To:" field is present, it
        indicates the address(es) to which the author of the message suggests
        that replies be sent.

It could be argued that Reply-To munging is not allowed by this, but I could still see munging as adding an address to a mailing list email seems a reasonable thing to me.

Also, the "Sender" header is meant for the example you gave (composing and sending an email on behalf of someone else), not Reply-To.