Impossible and probably counterproductive to enforce it; although gently documenting best practices is certainly worthwhile.
I read a wide variety of Apache related lists, and the style varies from community to community. Separately, some communities are amazingly consistent, and some vary depending on the writer. It's really up to the community as a whole to adopt a reply style; trying to change one that's well-established often turns into a religious war, so to speak.
Example: when you reply in-line, do you ...snip... the previous message to the relevant portions, or include the whole thing? Again, it depends if your community is used to reading the whole discussion list (where you should have already read the previous thread, so snipping saves a lot of time), versus being on some other kind of forum, where each message is only seen individually (so you might want to leave context there).
Personally I snip and reply in-line where it makes sense; i.e. to a specific set of points; otherwise I top post if I'm making a more general comment.
(Still deciding on travel plans: currently arriving Sunday AM, but thinking about paying to switch to Saturday AM to at least catch a few sessions... but hmm, would need hotel night too).