I asked PCA about just such a thing a year ago when I got my FJR and was picking up my Givi V46.
As I commute to work, and live in SoCal which means I can do that year-round, I was looking for a mount that woudl put the Givi behind, possibly even sideways (ie open to the side, rather than rear) to be narrower (lane splitting/sharing/whatever) and more aerodynamic. I didn't find anythign, went with the PCA sport rack and called it a day.
But, I'm still intereted. As the rack is held by only 3 bolts, it would be easy enough for me to swap when required if the solution was a completely different rack. I was thinking of a rack that mounted in the tail, and then came forward to maybe something like the side bag support point (only on multi-day trips do I use the side bags). I suspect one woudl be fairly easy to make for someone with access to a machine shop (especially if they had the PCA rack as a starting item to measure from)
Anyway, if you come across something, please let me know.