Okay, I can't find my past procmailrc files for some reason.

But I'm pretty sure this will work:

# add/override Reply-To for list mail
:0 fhw
* ^List-Id: .*<gnhlug-discuss.mail.gnhlug.org>
| formail -i "Reply-To: gnhlug-discuss@mail.gnhlug.org"

The "f" tells procmail the recipe is a non-delivering filter. The "h" means only check the header, and the "w" means check the exit status of formail in case things go bang. The "-i" switch to formail means "add this header, renaming any existing header if needed". So any existing "Reply-To" should become "Old-Reply-To".

I got the idea to add the "hw" flags from the results of a Google search just now; apparently many others have wanted this too. Which is why I think this will work; other people have posted the same thing.