Underworld should look no further than Nine Inch Nails for special edition inspiration. Check this out:
http://ghosts.nin.com/images/popup_product_deluxe.jpg
And that's not even the most expensive package.
NIN is leading the way with this kind of stuff for the independent artist. The music itself is essentially free, so give us something precious like a beautiful physical package or multi-tracks or a book. Underworld have cultivated a fanbase of aesthetically-minded creative types and we would eat something like this up.
It's also, I suspect, a license to print money.
Here is the most expensive NIN package, priced at US$300. They limited production to 2,500 and they sold every single one of them. That's US$750,000 before expenses. That's astonishing. I've no doubt Underworld could do something similar.