Of course you should permit to run such tests only if you are sure you are dealing with site owners or their representatives. You can use the system that is used in Google webmasters tools - users can confirm their ownership by adding specific document or meta-tag. Also - registration system's usability is not perfect. But the idea is great - I would use this service.
Yeah, this is something we have thought about also. We already have this security feature that uses a special file (loadimpact.txt) on the target system(s) to verify that it's OK to run large-scale tests. We should probably improve on that and allow people to bypass the other checks if they have this file. It is the safest way we have to verify that a test is legal, so we should give it more priority before other checks.
