Ah, I've found a viable Poser-native workaround, though not a fix. It's that Firefly renders can also be controlled by an official partner script.

Top Menu in Poser: Scripts, Partners, Dimension 3D - 'Render Firefly'.

At the foot of this Firefly rendering panel, there are the usual Auxiliary switches that are now missing on the standard Firefly settings panel. Since the auxiliary rendering features are still present, doing a Firefly render this way works fine - and saves a Photoshop file which has the expected extra layers.

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