This Canadian wildflower company includes lupine in their mix: http://www.wildflowerfarm.com/index.php?p=product&id=224&parent=3 ..which seems to indicate they are safe. But here it says lupine roots (at least seashore lupine) can grow to 1 metre long.. which seems a bit too long for a septic bed. http://www.pfaf.org/database/plants.php?Lupinus+littoralis
Well, Canadian variety lupines have a much shallower root system than seashore ones; it has to do with the composition of the soils. Also bear in mind that while those roots might be 1m long, they're not 1m straight down. Andean lupines, at least, have lateral root systems, so of course they're long, but they're shallow.
