>> Or maybe adding a defcustom 'diff-whitespace-style' would be a proper
>> solution here?
>
> Since `diff-setup-whitespace' unconditionally sets whitespace-style from
> a hardcoded value, this is the natural fix.
>
>> I started working on such a patch but stuck disliking the probable
>> need in the 'whitespace-style' large ':type' definition duplication.
>
> I think you don't need a duplication.  You can reuse an existing type
> definition by looking up the `custom-type` symbol property of an already
> defined option.  See e.g. "lisp/eshell/em-cmpl.el" for a few example
> definitions.

Thanks for the advice.

The proposed patch is attached.