It seems a 90 degree PVC* fitting could be attached to a roof plumbing vent and snaked out around a solar panel? It's just an air vent after all and you can make these bends in the 4-6 inch space under the panels.

Why is this not done? Code? It just seems silly to leave panels missing for a small plumbing vent. I don't see any technical problems with extending them as described?

Of course furnace and water heater vents are a different issue and cannot be re-routed like this.

*I say PVC fitting because ABS 90s have a larger radius than supply water fittings. This could be a space issue under a panel. Now we know that PVC cannot be bonded to ABS but who cares. It's just a vent and above the roof line so a press fit should be more than good enough. And the panel over it will help keep it from moving up.