If this malware is actually real then exploiting a large variety of different USB stacks (whether it's done via the the BIOS or OS stack) seems implausible. Maybe the flashed usb stick either:

1) Hides a bootloader on the devive that runs at reboot (assuming the BIOS allows it).

2) Pretends to be some kind of device (that most OS's have stock drivers for) that allows it to access main memory. Maybe it pretends to be a USB to firewire bridge (or something similar that gives it DMA).