Create a processed version of the raw audio that leaves sounds

above 1kHz unaltered, but reduces ALL sounds below 1kHz to the
amplitude of the ambient background (determined from a noise-free
section of the recording).

This is a drastic and unconventional thing to do, essentially
attenuating all sounds below 1kHz to the ambient level, leaving only
a smooth noise floor below that frequency. This is the task only DINR
can do, and I can't easily explain the settings required to anyone
unfamiliar with the plug-in (It involves manually setting a noise
floor curve, then using the 'Hi Shelf' function, setting the shelf
freq to the lowest, 20Hz, thus specifying all the higher freq
spectrum, and set it to attenuate, rather than the usual boost. I
usually run a couple of passes on the same curve, attenuating by
10-15dB each time, until an even noise floor is all that remains.
I've included a screen-shot of the DINR settings for this with the
accompanying files)