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.
They won't really care. None of them care. You just tell them you have a screenshots and few bits and bobs that'll let them offload the problem on someone else if it ever arises, and it won't. They won't even ask to see them.
This also even a risk to some 3rd world countries. Unlike those countries where bitcoin can be freely used. I guess this is the downside of taxless transactions. Some government banned bitcoin they though it will just bring their country into a bad demise. Also government cant really profit if that is the case. Some may say that bitcoin is some kind of tax evation but i think the only problem is dictating where you get your profit.