Checking on the official Toshiba Encore support forum (US, not the empty EU forums), it appears possible now to run the Encore from mains power whilst using USB devices (though it does not charge whilst doing this). I have a lead on order from Amazon and will report back. Link is here:
USB & Charge maintain success - TOSHIBA FORUMS
Have fun folks.
USB & Charge maintain success - TOSHIBA FORUMS
Have fun folks.