I realize I'm taking a huge shot in the dark here (or maybe it's better described as "grasping at a straw") but might it be possible to connect the USB port of an SD USBPre 2 to the USB of a MixPre 3/6 and record the USBPre's 2-channel output in the digital domain?
My guess is, it's not possible.
But I'm thinking how great it would be to have both devices in the same bag — 6 Sound Devices preamps and limiters — and avoid analog cabling between them.