At least one message is needed to send the kill. So that leaves one message for coordinating said kill and everything else...
Like you said day talk is a plus but much at all is going to be a tell, imho.
I am anxious to see how this plays out.
I was going to point out that you don't have to say publicly who you're communicating with (you can just post an encrypted message that everyone has to decrypt, but only one player will be able to successfully decrypt it), but I think you run into trouble then if everyone posts publicly whether or not they were able to decrypt your message. Your scum buddy could deny it as well to stay hidden, but then you look very suspicious for having a communication with someone who won't admit that you're communicating with them.
This has the potential to be really cool, but I think the roles really need to be very cleverly designed to maximize the meaningfulness of private communication and signatures. I think the much bigger issue is that it will be very tedious and people will probably lose interest quickly because of that (but I'd love to be proven wrong).
Also, if there are powerful PR's, you may not need to require that town post at night. It might be interesting to see what happens if no one is required to post; VT's might want to post so that they look like PR's and draw the NK's, or scum might want to deliberately not kill so that they look like VT's (though that only works if other PR's are posting).