If I were going to implement it... hmm...
I think I'd just special-case the genesis block, so it has a value out of, oh, I dunno, 20 million CompleCoins. Then maybe fix the mining reward to something small and constant (say 0.001 CompleCoin per block, forever-- or whatever you like).
The central authority would create the genesis block, and so would have 20 million CompleCoins that it could issue (aka spend) however it pleased.
The risk would be the central issuer ever losing control of its wallet/private key. That risk could be mitigated a little bit by occasionally changing the key by spending all of the non-issued coins to a new address in a new, secure wallet somewhere and waiting a few blocks for confirmation.
I'm having lunch today with somebody else who is interested in using bitcoin tech for a centrally issued alternative currency, so I've been thinking a bit about it...