If you do not "own" the end-to-end network infrastructure, there is no way to guarantee any preferential handling of any particular subset of traffic.
There is a way to guarantee end-to-end QoS even if you don't own all networks along the way. That is for a 3rd party organization to impose a contractual requirement. Aside from the very common scenario where a large customer contracts with 2 ISPs, there are some 3rd parties which require such QoS in order to "certify" an ISP. In Europe, the automotive industry does this. Maybe the ANX does similar? Maybe one of the ANX certified service providers could do a talk about their experiences? If a group of ISP's want to create a trade association for end-to-end QoS guarantees, they are free to do this. Perhaps the shift to VoIP will even make this economically viable. In any case, this issue should make for an interesting panel discussion if NANOG does a VoIP track/theme. --Michael Dillon