Can you provide a code citation for that? I've only been able to cite CPC 310.5, but I feel like that's a stretch. Manufactures installation instructions sometimes specifically prohibit this.
310.5 would plausibly apply if the panel is so close to the top of the plumbing vent that the net free area between them is smaller than the interior area of the vent pipe.
If you do the math, when the end of the plumbing vent is cut square, the vent is plumb, and the panel is parallel to the roof plane, even when the panel is touching the vent at the downslope point, that can't happen if the roof slope is 6:12 or greater. And that answer is for an arbitrary pipe diameter and pipe wall thickness; for real world pipe sizes, the maximum slope at which you could obstruct the vent is even lower than 6:12.
And even for a flat roof, you'd just need to raise the pipe 1/4 its diameter above the end of the vent to get sufficient free area. Although in that case you may well not be installing the panels parallel to the roof plane.
Given that, CPC 310.5 is almost never an issue. The only issue I could see is CPC 906.1, which says the vent needs to terminate at least 6" vertically above the roof. If your racking method raises the panels high enough to allow the vent to still comply with that, I see no general prohibition on running panels over a roof plumbing vent.
Cheers, Wayne
