When joining a reports-enabled server, provide a DHT-based "backlog mode". Basically the ability to see past chat messages from before the player joined. All powered by a DHT, with no central server, entirely client-side, and without other players' consent.

What you mean by "DHT"?

Probably distributed hash tree. Since I think messages do some like weird chain signing thing now it should be possible to authenticate backlogs based on the first signed message you receive after joining (or right off the bat depending on how it works idk)

In my opinion this falls onto "exploit" and "proof of concept" territory so even if implemented, it should be separate from the main NCR mod.

Definitely not exploit, but indeed complexity of implementation does little to justify inclusion of such feature in the mod that focuses on eliminating message signatures, not putting them to some kind of use.