I know that you intend to limit the risk by restricting source addresses and of course by getting consent of the probe hosts. I am just afraid that even asking the question will make some hosts doubt that we will not harm their networks and their connectivity. We have to realise that not all the hosts will share our enthusiasm about new experiments and the more hosts we will need to find, the less of them will be as open minded as we are ourselves.
I agree that atlas seems like a perfect distributed system to test BCP38 compliance and help operators secure their networks by providing a simple tool to do so. I agree that getting agreement from individual atlas hosts and being careful about the addresses chosen to spoof with is the right model to consider. Perhaps as a first step atlas hosts could be formally surveyed about their willingness to allow spoofing tests from their networks. If any hosts have reservations along the lines of "not harming their networks and their connectivity" then this should be addressed. But at the moment we don't really know what atlas hosts think about allowing spoofing tests. At the same time, Atlas seems like a great platform that could make a substantial contribution to reducing the ability to spoof and the harm that comes from it.