I've had a play about with this today and I've worked out what looks like the way to do this, it seems to have worked for me...
Exactly as suspected the files go in a vuplus/solo directory from the root of the USB stick. You then put the USB stick in the front USB port on the SOLO, then turn on mains power and you should get:
ORANGE/RED LIGHT (CONSTANT) IN ROUND DISPLAY WINDOW - LED LIGHT FLASHING ON USB STICK (if yours has an LED)
after a min or two you'll then get
ORANGE/RED LIGHT FLASH IN ROUND DISPLAY WINDOW - NO FLASHING FROM LED ON USB STICK
then just a short while later
GREEN LED FLASHING IN ROUND DISPLAY WINDOW - NO FLASHING FROM LED ON USB
That's it, you can now remove the USB stick and turn the mains power off then back on. Once you turn the power back on again the green LED in the display window should be constant and it'll just be a black screen on the TV, you must wait a good while now, longer than you'd imagine, then eventually the VU+ setup wizard screen will appear and you are good to go!
Seems fairly obvious really now that stage one is transfer info from USB stick, stage two flash from memory, stage three complete! Hope this helps.