In this case, you are going to need to explain the origin of your coins, and im not sure how I would explain I made some BTC posting in some internet forum, this may not be convincing to them even if it's the truth.
But it is the truth.
You should hopefully still be able to prove that you own the account that posted. You'll be able to point to the thread where you signed up. The thread will be there that describes the sig campaign and the pay rates. If you're sensible you'll retain the private key of the address that received the payments and lists each tx.
I can't imagine tax authorities asking you for your Bitcointalk login or private keys. They certainly shouldn't, and technically that wouldn't prove ownership of the funds at the time they were paid. I'd archive the sig campaign threads in case the site gets nuked, but that's more about proving where funds are derived from and less about taxes.
I think the IRS really just cares that people are making a good faith effort to pay their taxes. It seems like signature campaigns would fall under regular "wages/salaries/tips" or "other income" and taxed at the standard tax rate for the cost basis, and capital gains for any gains from holding.