Easier than that. If I provide you with a deterministic npub, you can have a DM client that uses a new one each message and it would know to keep all those in one chat.
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And the initial npub trade can be done over normal DM channels. It would be similar to payment channels at least as far as the UX is.
Click a button "Open Ultra Secret DM" with a user. Deterministic npubs are exchanged, then chat commences using 1-time npubs.