As for the whole 0.5V thing, they're actually rated above that but I figured (like you said) that the sun won't always be at full power.
Also, the last solar panels I purchased were rated well below what they could output. (Not saying that these will be able to, just saying the ratings aren't always correct)
If you measure the open circuit (=unloaded) voltage, it will be higher than when loaded appropriately.
Beware of the specs, some sellers give open circuit voltage and short circuit current and just multiply them for the power rating and that's bogus, as they cannot happen concurrently.
You cannot use the short circuit current rating, as this only exist when the cell is... Well short circuited.
And the open circuit voltage won't be seen with a load.
As far as using several in series, that isn't really an option at the moment for several reasons. I'm just going to keep surfing around wikipedia until I find something that can work.
Adapted joule theif?
If you're referring to the real Joule Thief (not the one there's been a commercial for here, where the name has just been "stolen", there is only one Joule Thief, as the term is coined by Big Clive who first named the circuit so, although he didn't invent the construction), you won't get there unless you have some germanium power transistors (which probably haven't got the gain required anyway).
Best result I have gotten from such circuit is down to slightly above 0.5V and only if the oscillator was started at a bit above 0.6V (how much is depending on your transistor selection) and at such low voltage, the allready not too efficient circuit is very inefficient at such low voltage.
A general rule in switchers: The larger the difference in in/out voltage potential, the less the efficiency. (And that's why a SEPIC converter with an output at close to the average input voltage is aces here).