How is the dollar or any currency used as a peg currency?
I don't see why a country couldn't just say "Our currency, the fubar is pegged at the rate of 10 fubars to the bitcoin." Anyone needing to conduct transactions in fubars would have to then buy 10 fubars for one bitcoin.
The only way this works, though, is for the government to restrict transactions in other currencies and only allow the national bank under their control buy foreign currencies. I think some get even more draconian, forcing travelers to trade all their foreign currency at entry to the local currency at the official exchange rate and then sell you your own currency back at exit.
Of course this doesn't stop black markets for currency, every country with an official exchange rate has them, more so if the official exchange rate is greatly detached from reality. But these can be dangerous and you may find you detached from your freedom.