I recently purchased New Trent iCruiser IMP1000 11000mAh External Battery Pack and just to let you all know that I tried and succeeded charging my TF!
I took 2,5 h to charge TF full from 50% battery but the only trick is that the TF screen must be off. While connected, TF didn't show any signs of charging, like battery icon or similar but to my surprise it did fully charged.
iCruiser has an output voltage of 5 V, 1 mA so using it with TF is similar to usb charging trick. Usually while I charge my HTC phone with iCruiser it goes off when finished but with TF it stays on so you have to turn it off manually.
So if anyone owns or plans to buy similar or same external battery pack, it could save you while on trip etc.