I was proposing to value Bitcoin by taking an average from all the data we can from an exchange; LibertyReserve, any fiat currencies traded, even gold.
How can you prevent the exchanges from manipulating the data? Even if you take the median rather than the mean, this is very much centralizing the difficulty adjustment.
"alt currencies generally rely on bitcoin for value"
Hang on, does that mess it up because that would mean it's feeding into itself?
A P2P exchange can be set up using more than one cryptocurrency through a bitcoin-like system, which will allow taking bitcoin's price over a large amount of alt currencies. This is very unreliable because such volume would likely be so small any person with ten grand could manipulate difficulty greatly.