As you all know, a few (loud) users still seem to think reply-to header munging is a good idea, and thus I and many others feel forced to enable it on certain lists. This causes problems every time people would like a reply off-list, since people tend to reply to the list by mistake, obeying the munged reply-to.
Turning off first_strip_reply_to looks like it would help at first glance, but it still requires users to edit the headers manually to remove the added list address, and many forget this.
A simple solution would be to simply allow leaving any existing reply-to-headers alone, and only add one if none are already defined. This way, the munging will act as a default, allowing the sender to override it by adding an explicit one.
The change should be fairly simple, and something similar to the attached diff should suffice. Naturally, the setting "leave_existing_reply_to_alone" would also have to be added and documented. The diff is just to illustrate the idea (although I can create a complete diff if you wish).