Reality check: Most mailers today already theoretically give users some kind of option over where a reply goes. The problems arise because in the real world, users pay no attention to that stuff. They barely know how to hit Send.
So what in the world would delude us into thinking that if we introduced some kind of per-member Reply-To: option, these people would know or bother to use it? They'll never touch it! The only people who will change it are the people who are already savvy enough to direct their replies correctly without it.
Also, on lists of the COMET-ANNOUNCE-L variety, where the admin has specifically set a reply policy or reply address, it seems positively harmful to allow individual members to override it.
I do not see the usefulness of this per-member patch.
The per-member patch that I WOULD like to see added is member_aliases - a list of alternate addresses under which a given member posts. "Address creep" is a daily plague, and it's not adequately answered by creating nomail "ghost members" or stuffing the accept_these_nonmembers pool, because when people leave things get out of sync. If the per-user options screen allowed both user and admin to edit an alias list, and if we wrote a little add_member_alias script, things would go a lot easier.