Not really, sorry. The KB article linked to above states that dual channel USB was introduced with the slot-loading iMacs and the PowerMac G4 (AGP Graphics). Unfortunately there is little documentation about which models sported it and when it was discontinued. That said, I vaguely recall that it was around for at least one year.
Btw, you may be able to
Boot PowerPC Macs via USB 2.0 drives as well.
