There already are several alternate currencies built using Bitcoin's code, each using a separate blockchain. So alternate bitcoin-type currencies are not hypothetical, they already exist.
For example in the bitcoin IRC channels I have a sample IRC bot that supports a number of these alternate currencies.
The way the bot currently works is each currency has an account with each of the other currencies. For example the Brits have a Botcoin account and a Canadian Digital Notes account and a General Retirement Fund shares account and so on.
This has the effect of each currency having balances of various other currencies it can use to "buy itself back" aka to redeem its coins.
If for example someone (such as the Canucks for example) had bought some Britcoins using Canadian Digital Notes then Britcoins would have a balance of 100 Canadian Digital Notes available to be used as "backing" for Britcoins, that is, available to be used for the purpose of buying Britcoins by spending Canadian Digital Notes, much as the Canucks already did to cause the Brits to have that 100 CDN on hand.
The bot keeps these backing balances separate. That is, it will not spend any more than the Brits actually have of each other currency/commodity on buying back Britcoins.
This is looking to be an interesting market, because of course once someone has managed to buy those (for example) CDN from the Brits in return for Britcoins, then that is it, the Brits have no more CDN left with which to "back" aka "redeem" Britcoins.
In other words the bot does not lump all CDN together, it tracks separate balances per each other currency, so CDN it obtained by selling Britcoins will only be used to buy Britcoins, not to buy for example Martian BotCoins. Martian Botcoins would have their own distinct CDN balance, consisting of CDN that had been used to buy Martian BotCoins, and only those CDN would be used to buy Martian BotCoins.
That simple mechanism makes the bot in effect act on behalf of each currency as an administrator of a "backing facility" providing "backing" individually for each currency.
These currencies seem especially well suited to situations in which moving of hard goods is awkward, such as when remote operators operate robotic/waldo facilities in remote galaxies so far away that only consciousness/telepresence/data can reasonably be shipped.
Instead of trying to ship you a hundred thousand tonnes of deuterium from, say, a galaxy in the
http://galaxies.mygamesonline.org/ group of galaxies to, for example (though maybe forbidden by Zorg's terms?) a galaxy in the Zorg Empires group of galaxies, I could sell deuterium in one group of galaxies for digital coins then use those digital coins to buy deuterium in your local galaxy to ship to you, thereby saving shipping costs and thus maybe provide you with deuterium cheaper than if I shipped it from a galaxy far far away...
-MarkM-