I know there probably isn't much on offer?
A font chooser for FLTK fonts, like the one in fluid?
Or a font chooser for the local OS like in test/fonts?
Or something different than those?
No, there’s no fltk font-chooser widget as such.
That sort of convenience widget is somewhat outside the usual scope of fltk - though the bits you need to write your own are all there, of course.