I's be rather surprised if anyone ever did. F/C is a static input, not meant to be switched at runtime. There are not only no examples for switching, but it's as well clearly stated on page 3-392 of the manual:
Strapping is datasheet lingo for fixed setting
Looking at the (pseudo) schematics reveals that it's not meant as an active switcher as there is no protection against illegal timing states (glitches) during switching.
Using F/C does not make sense.
In general, there are essentially only two uses for varying clock speed:
- Slowing down (Switch between different speeds)
- Following a more sophisticated external scheme - like fitting a video timing (*1)
For slowing down it's usually more appropriate to use wait-states on the CPU itself, as this leaves all (internal) timing unchanged.
For a more complex timing CSYNC could be used - but when using CSYNC, an external source on EFI is required anyway, making the internal obsolete again.
Long story short, use an external source via EFI when interested in more complex clock solutions. In fact, such may as well make the whole 8284 obsolete, as its features may be added easy or come as side effect.
Having said that, it still might be possible to use the oscillator (OSC output with F/C = LOW) to feed an external clock generation, to be fed back via EFI, but it'll need some research (*2) before using it in any production setup. Not at least for noise reasons.
*1 - The Apple II is eventually the prime example here as the CPU clock is wraped around the needs for video.
*2 - And talking to Intel (or whatever the prefered supplier is) to make it part of the (your) spec for parts.