Just cruising around the Service Honda site today and stumbled upon this:
That, my friends, is a FMF Gnarly on a KX500AF. FMF won't make Zip-Ty low pipes any more, but they're making these.
The expansion chamber is low enough for sure. Looks like it is too low to meet the KX500 muffler though. I bet it wouldn't be too hard to cut off the last taper and route it up.
It is a tight fit though. Look how it touches the radiator hoses.