“it’s worth zero so cost basis is zero” <<< wrong.

Until first verifiable sell happens, you have a NULL cost basis for ETHW, which cannot be used.

The moment you have receipt, the chain is live at the same time, in the first millisecond $0 does not exist because market data does not exist yet. Your cost basis cannot be null. The market will create pricing data within milliseconds, when the first verifiable sell/buy transaction occurs. Further more, the IOUs, sort of a futures market, will also provide data that may or may not be needed. So receipt will happen milliseconds between the IOUs market pricing data, and the first verifiable sell/buy transaction that uses the live chain. THERE CAN BE A NON-NULL NON-ZERO PRICE THAT CAN BE ARRIVED AT for tax purposes, based entirely on the first verifiable transaction. Unless the IRS provides specific guidance, that's how I'm likely going to arrive at cost basis for ETHW.