Ok, I found the first little bit of magic (our manual is honestly kinda rubbish in describing the denoising features and there’s out of date information hiding in various places).
If you have the Denoising Data pass set turned on:
You will find that in the Cycles Render Properties -> Sampling -> Denoise section that even when the checkbox next to the Render checkbox is OFF, that the drop-down list for choosing the denoiser is ACTIVE, and you NEED to select OpenImageDenoise as this setting controls WHICH passes are generated as part of the “Denoising Data” passes. When you do this then you get just the passes needed for OIDN and the render is much faster.
Edit: It seems like it’s the “Noisy Image” channel that is affected by the denoiser choice in this case. As long as you stick to using the Image channel as the input to the Denoise compositing node then you may never notice, though the NPM denoising passes dramatically slow down the render which I suspect is affecting many people without their knowing it. Also the tile-based denoise options (Render box checked on for the denoise option in the render sampling properties) are all terribly slow compared to using the OIDN as a one-shot compositing operation.
Still playing. I may bug some of the behavior and/or documentation on this.