I have seen a few as well, but people would have aftermarket cross bars on roof rails. Because the aftermarket cross bars, you can space any way you want. I don't know if you can do that with OEM ones, that's why I am asking if anyone has mounted the box on oem cross bars.
I believe you can change the spacing on the oem crossbars. I noticed a few mount holes but I havent tried it myself. I'll do so as we get closer to winter since my snowboards weren't positioned very well last season.
You can move the rear cross bar to one of three locations. The front bars location is fixed.
You are correct though the movement of the bars is really restricted and therefore less flexible then aftermarket bars. The bonus of the OEM bars is they look really nice, and blend in with the car, whereas aftermarket bars often look pretty bad.
As far as the box goes, the bars will support it, but placement may be an issue depending on the flexibility of the feet on the box.
Thanks for clarifying that for me. I am trying to find a cheap box that fits 4 snowboards, seems hard as those things are really expensive.

i have the Thule boxster with thule rails and i love it. easy to mount and holds alot.Iery aerodynamic so wind noise is very low. I have been through many options for snowboards from yakima fatcat 6, to a long box thule. I would never go back to ski rack.
in one of the links pornandbeans posted I saw your post and it linked me to your thread. I think you said the box was making a lot of wind resistance that you can actually feel and a lot of noise. Is that the old thule box you had?