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.