I'm certainly no expert in crypto, although I think I have a basic understanding.
There is a FAQ on crypto here, which refers to Notice 2014-21:
FAQ:
https://www.irs.gov/individuals/international-taxpayers/frequently-asked-questions-on-virtual-currency-transactionsNotice 2014-21:
https://www.irs.gov/pub/irs-drop/n-14-21.pdfBased on a quick scan it seems that your question has not yet been addressed. You may want to ask the US Treasury for guidance as indicated at the beginning of Notice 2014-21.
The general problem with your situation is that the value of anything is normally defined (in the US, currently) by the fair market value, which requires a (public) market, which by your question doesn't exist. So the IRS doesn't really know the value of your crypto, so they therefore don't know that it's worthless.
With more traditional assets like stocks, there was (and presumably still is) a process where a brokerage firm would "buy" back the stock at $0, which would simultaneously relieve you of ownership and establish the value so you could take a tax loss. By the nature of crypto, there isn't (yet) such a similar service. Perhaps some business or crypto speculator will take on that task in the future, since there is a bit of a market need.
You could take a more cowboy approach and try to meet the definition of a sale here:
https://www.irs.gov/faqs/capital-gains-losses-and-sale-of-home/losses-homes-stocks-other-property/losses-homes-stocks-other-property-1And then report the "sale" on your tax return and take your chances with an IRS audit. I don't know the details of crypto transactions, but maybe your original idea of selling to a NULL address is in the ballpark. My guess would be that the design of crypto would prevent such a transaction, but maybe not.
The most conservative approach would be to contact the Treasury Department and ask for guidance as mentioned above.