...then 5 BTC can cover 50k * 10k = 500 million gold
The current gold cap per character is at 1 million - 1copper
I don't think Blizzard releases stats on how much gold is in circulation on each server but it might be possible to scan auction houses for bids and successful buyouts for example as a minimum baseline. I believe however that on one server there is FAR more than 500 million gold distributed between characters.
Let's assume that there are 5 billion gold coins stored in player accounts on an older, big server, these could be 1:1 mapped to 50 BTC.
According to
http://www.warcraftrealms.com/dbstats.php there are ~500 servers with about 100 million total chars (who on average would own just 50 gold each)
Just to 1:1 map all their gold to satoshis, Blizzard needs to own 25k BTC.
This does still not solve the issue that in this model 1 gold would be worth 1 satoshi then, while currently it is worth 20k satoshi --> if you open a market where players can buy and sell their gold for BTC (or even exchange at a fixed rate) you either promote selling gold for USD still (you get 20000 times more money if you sell 10k gold for 11 USD) or crash the economy (it is 20000 times cheaper to buy gold for BTC than to farm it yourself using the cheapest slave labour/bots possible).
All in all my points are:
1) 5 BTC are probably NOT enough to 1:1 map all the gold on one WoW server.
2) If Blizzard wants players to be able to exchange gold to BTC and vice versa at fair prices and be a market maker in this, they need 25k*20k BTC - far more than ever will exist.
3) If Blizzard just wants players to get a token amount of BTC as a gold sink(?) they still need half a million USD to give away to players(!) at current prices for this little project. It's far easier and cheaper to invent something else as gold sink.
4) Transaction fees... "I want to sell gold for 20000 times under market price for these Bitcoins and then above THAT I have to pay transaction fees that can buy me half a continent ingame?!"
Sorry, this just doesn't work out. Neither for the players, nor the gold farmers nor for Blizzard.
There is a minecraft server though for example, where you deposit 0.1 BTC for signup and then can earn some Bitcoin back by playing (50 satoshi per block or so).