It should be possible to tell OIDN that the guiding passes are noise-free without prefiltering the guiding passes. See the enum from Cycles for reference:

enum_denoising_prefilter = (
    ('NONE', "None", "No prefiltering, use when guiding passes are noise-free", 1),
    ('FAST', "Fast", "Denoise color and guiding passes together. Improves quality when guiding passes are noisy using least amount of extra processing time", 2),
    ('ACCURATE', "Accurate", "Prefilter noisy guiding passes before denoising color. Improves quality when guiding passes are noisy using extra processing time", 3),
)

The NONE prefilter sets cleanAux to true, so that the OIDN preserves details the best when the guiding passes a noise-free.
The FAST is how the OIDN works in the current master branch: guiding passes are denoised filtered together with the color pass. No prefilter is requires, cleanAux should be false (or not set).
The ACCURATE prefilters the guiding passes, and sets cleanAux to true. Is slower than FAST, but preserves details better when guiding passes are noisy.