you can try this without any fear.
afaik that person lost it's coins by overwriting the wallet-file at some point (simply do a backup first!), not by trying to double-spend coins.
Actually, you can lose coins this way. Consider - you have 2 BC (as a single coin) and spend 1. Now you have 1 BC as a single coin
with a new private key. If you restore the old wallet.dat, your new private key is gone. Once the client realizes the 2BC coin was spent, that coin is deleted, and you're left with 0BC.
Ooops! Yes! You're quite right if the PGP key is compatible with the key type used in BitCoin. I still associate PGP with RSA and I believe BitCoin uses ECC so I was thinking you'd have to update the software to understand RSA keys.
A production version of BitCoin will probably have to accept a number of different Public Key algorithms.
ByteCoin
Right, I thought we were talking about a private key that was the same type as the client (and assuming the client uses some known standard). Obviously it wouldn't work if you were to provide an unrecognized type. =P
However, changing the client to recognize multiple types of PKA's WOULD most likely be a breaking change, unless satoshi is including some kind of version information about what keys were used in the block chain.
It would probably be easier to add a extension field to GPG to list bitcoin addresses. Note, however, that the destination address (actually, the signature acceptance script) is visible to everyone, so it would be a very public transfer.
That said, however, there is support for partial signatures. That is, you can create a transaction from yourself, with a signature that excludes the destination fields. Now you just encrypt that, email it to the recipient, they add on said destination fields, and there you go. This, however, has the issue that other nodes can _also_ modify said destination fields, and the first one to get in the block wins. I'm not sure if there's some clever fix satoshi had in mind for that...