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.
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.
Being someone who makes a career out of advising clients on US tax and compliance matter, the IRS is aware of a lot of these payment arrangements and is making plans to ramp up compliance. One article I had read reported a contract that Chainalysis won in a public bid with the IRS to begin analyzing cryptocurrency non-compliance. As always with the government, technology lags far behind. But not reporting income accurately and paying tax is unlawful. "Intentional disregard" for tax laws is criminal.
As for BTC won in sig campaigns, it would be Ordinary income. Depending on other factors (time involved, activity, etc.) it may qualify as business or hobby income, both of which you could use related expenses (computers, subscriptions, office space, etc.) to offset income. Business income could have sufficient expenses to claim a tax loss, but net business income more than $500 is subject to an additional 15.3% self-employment tax. However, with proper planning, a lot of the potential headaches in compliance can be avoided. In fact, if this is regular, substantial income, there can be a lot of great tax planning opportunities to tax advantage of.