I have the early Moto Guzzi accessory round carbon fiber (Mistral) on my 2000 Sport brackets mounted to the inboard side of the hangers. Those hangers have also accommodated Tekno pannier brackets all these years. So, not so much of a "standard" as an *example* . . .
This was an effort, on my part, to "tuck the pipes in" (they don't) and make them look more symmetrical (they aren't).
Once you have them where you are happy, loosen every fastener from the hangers, center hanger (crossover), pipe clamps, and the header flanges at the heads and tighten them back down in a "star pattern" like you are tuning a piano.
Make the Heavens ring!