I see how technically with -privatebroadcast=1 a client wallet could simulate a third-party batching their dust while doing it all themselves. And doing this could be a bit more block space efficient and relay efficient. But I’m more concerned that in an environment where dust disposal tx are few and far between an attacker could easily correlate dust UTXOs confirmed and/or broadcast around the same time. For convenience I’d rather see wallets pre-sign dust disposal txs and then broadcast them at random times, and/or when they see some others in the mempool they can batch with.
I’ll try to improve the rationale section of the proposal around this point.