This is a proof of concept that on iOS you can override the systemFont(ofSize:) and preferredFont(forTextStyle:) with a bit of swizzling.
Would also be possible with some ObjC code that arrogantly overrides the existing methods in a category of UIFont.
The one thing that's still missing is to override fonts set in a Storyboard when it's using text styles instead of System Font (of size x).