The hot key API does not support the Fn key, nor does it differentiate between left and right modifier keys.
Remapping the Fn key can be done with low level tools. Maybe one day Keyboard Maestro will include low level remapping of keys, but it does not currently. Low level key remapping is really below the level that Keyboard Maestro operates.
This is borderline rude.
There is no such thing as “saving it for a later version” - my todo list is a mile long and has thousands of entries, one of which is low level key remapping. The fact that it hasn't been done is not “saving it for a later version”, it is “other things have had higher priority in my view, which at the end of the day is the only view that matters for what I implement”.
And “they're lazy” is just plain rude, and if you persist with rudeness, you will not be welcome on my forum.
Keyboard Maestro does not implement things that are not available with public APIs, which is why Keyboard Maestro back to version 4 (released over a decade ago) runs on Apple Silicon Macs. That's not lazy, that is a design choice and careful programming.
Keyboard Maestro does not solve all problems for all people, nor does it implement all features everyone wants, or even all features I want. That's called finite resources.
hidutil was not released in 10.11 which is what Keyboard Maestro 9.x supports. hidutil is interesting, and Apple describes the code to do it directly in TN2450, but hidutil and the method described there only remap a key to another single key, which, while useful, does not (at least as far as I can tell) provide the ability of Karabiner Elements to map Caps Lock to the Hyper Key (Command, Control, Option, Shift) which is what appeals most to Keyboard Maestro users generally.
So you can use hidutil to remap the Fn key to Control, for example, and then use Fn/Control as a hot key modifier, but it would conflict with existing uses of Control, not provide any new space for hot keys (as the Hyper Key effectively does since pressing all the modifiers is rather cumbersome).
So yes, Keyboard Maestro might support the hidutil remapping in the future, but since it does not actually provide the Hyper Key facility, it is a relatively low priority, and since you can already do it yourself using hidutil, and using an Login trigger, and it does not interact with any other Keyboard Maestro features, there is even less reason for me to implement it.