The Brits and Canucks tried this with United Kingdom Britcoins (UKB) and Canadian Digital Notes (CDN) respectively.
( See
http://galaxies.mygamesonline.org/digitalisassets.html )
Their theory was that actually "backing" coins by retaining everything they sold them for to use as "reserves" with which to buy them back would work out much better than trying to "back" coins that random miners all over the world were mining and dumping without offering to buy them back.
However they also used the bitcoin approach of only ever having 21 million coins in total.
As a result, other nations and corps that were not trying to "peg" their own coins to fiat tended to take advantage of the "low" prices the Brits and Canucks tried to maintain; basically whenever the Brits and/or Canucks sold off their own coins "cheap" to try to prevent their price rising higher than the fiat they were trying to "peg" to, others, particularly the Martians, would snap them up.
So I guess for starters you will need a coin you can mint more of as needed, so that any time its value tries to rise higher than the fiat you are hoping to "peg" it to you can mint more to sell to try to bring the price down to fiat-parity.
The Mastercoin project and the Bitshares project have ideas they plan to try for "pegging" somethign to fiat, have you looked into those?
-MarkM-