I love the idea but unless you can profile what available capacity the probe/anchor has I don't think the resulting measurements will be useable. There is no way to know your http request was slow because someone with the end point is a hard core torrent user maxing their location out. Also valid for the areas where you have a hard limit and minimal extra bandwidth. ping/traceroute while not always a good test does squeeze through with minimal variance in result when the site bandwidth is congested.
also as an anchor host, can I limit the max bps because some locations is not low cost if everyone decides to http test some speedtest file. Our singapore anchor for example would cost more per month then we spent on the hardware to host an anchor in the first place. I suspect the probe/anchor hosts in other areas like africa, australia, new zealand, and south america would get even larger monthly bills.