I've since realised that the answer to my original question is a yes, as Coinjoin couldn't work if it wasn't possible to sign one tx with multiple private keys. So it can be done, I just don't think Armory now lets you as a part of it's transaction building GUI. A complete dust-be-gone feature would be nice though, and it'd be especially efficient if it allowed many wallets to one transaction 
Strictly speaking, the answer to your subject line is: yes. The new simulfunding features do allow you to simulfund one transaction from multiple input wallets. However, talking about dust: due to the complexity of allowing it, we do not have coin-control available for simulfunding transactions, so your wallet would have to be empty except the dust, in order to guarantee that the dust is used in the simulfunding tx. If you have 5 wallets with only dust, you could create 5 prom notes for the full amount of each wallet -- which can be done by going to "Merge promissory notes" and then add 5 notes in the interface, one from each wallet. But if the wallets aren't otherwise empty, it's likely not to behave how you expect. Though you could just empty all of the wallet entirely back into one of your own wallets, which will destroy the dust and merge all the wallets into a single address/UTXO.
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