I bought a USB Pre 2 to use on a gig this week, it was an excellent tool.
As for getting USB 5v power to the unit...
I purchased a hefty USB power source, the iSound Portable Power Max (16,000 mAh)
The outputs are rated up to 2.4amps, well above the USBpre's needs. This power supply is almost as big as the USB pre 2 and attached nicely on top with velcro. I ran the unit for a good 5 hours and barley drained the iSOund battery, I expect you could do several days of recording on a single charge. iSound sells a half sized 8000 mAh unit as well which I expect would also be great and be about half the weight.
You could also likely power your recorder off this unit for extended periods if its takes 5v power in. I think the Zoom H4n runs on 5v, but I dont think the Zoom has SPDIF in. Because, ideally to really take advantage of the USB pre2 it may be best to record the units digital output.
A further question:
I am studying options for recording the USBpre2 onto a portable recorder.
The only two small recorders with Digital Input that I am finding currently available are:
Sony D-50 (with toslink digital in) http://transom.org/?p=82
Marantz PMD661 http://transom.org/?p=1774
and then there was also the discontinued M-Audio MicroTrack, which was a lowcost product (with reliability issues) but it did have spdif in.
IS ANYONE USING THESE RECORDERS WITH THE USBpre2?
I am not thrilled about having to use Sony's memory stick storage or using the recorder itself to offload (instead of just pulling common format cards and popping them into a reader.) The digital in connector also seems like it could be an issue. But Sony makes 'em pretty solid in general.
The Marantz is bulkier (not looking forward to getting it and the USBpre2 and battery all laid out in a working bag set-up) But on the plus the Marantz is very old-school solid, easy to see whats going on, and has a nice simple spdif in.