Assuming that there is such a thing as a "list preference" for
replies to the list, that can be easily achieved by haying the
list expander insert a Reply-To field pointing to the list
mailbox if and only if there is no (author-provided) Reply-To
field in the message.
Only if authors somehow learn to always insert reply-to when
they don't want the list to override it. Even then, there's
potential for confusion among recipients who might be willing
to override a list preference, but not an author preference.
(or vice versa).
It doesn't require umpteen different
fields which modify the semantics of other fields or which
modify the behavior of the *recipient's* UA. Some list
expanders do insert Reply-To fields; the contentious issue
seems to be ignoring the "if and only if" part that effectively
causes such list expanders to override individual preference.
There are fewer problems with lists that only insert reply-to
if it isn't there already, but some problems still remain.
(see http://www.cs.utk.edu/~moore/opinions/reply-problem-list.html ,
items B3, B4, and B5)
Keith