Couldn't you use the cost of operation? I figure you could either keep track of it yourself or use a standard depreciation rate on equipment and electricity costs (since probably few people tracked that). Would be like running a business, which you kind of are as a miner. But you do make a point that this is a difficult thing and coinbase can't really track it.
BUT from what I understand, people leave a lot of coins on exchanges and never transfer them back to personal wallets. Coinbase can track that.
"Couldn't you use the cost of operation?" - that's pretty inconsistent with other IRS rules... you probably need to convince the IRS you're mining bitcoin as a business with the intent to profit with all the proof that entails, and then it's likely deductible as a business expense, not a cost basis, similar to how margin interest is not part of your cost basis for buying a stock.
None of that covers cryptocurrency directly yet, but looking at how more traditional investments are treated can inspire you to try to do your taxes in the spirit of the rules to avoid likely problems.
BUT from what I understand, people leave a lot of coins on exchanges and never transfer them back to personal wallets. Coinbase can track that.