Ultimately though the issue wasn't the tax, but that you had to register the evidence of your crime. If you could pay the tax anonymously it would have been fine. On the flipside, merely a requirement to register without paying the tax would still be self incrimination.
The form 8300 actually requires you to state both counterparties of the illegal transaction, which goes well within furnishing evidence useful in incriminating yourself. It's far more incriminating than the annual tax return, which shows an aggregate yearly amount rather than granularity of single transaction (or collection of "associated"-transaction) along with the date and name of both counterparties and a host of other details.
The form 8300 actually requires you to state both counterparties of the illegal transaction, which goes well within furnishing evidence useful in incriminating yourself. It's far more incriminating than the annual tax return, which shows an aggregate yearly amount rather than granularity of single transaction (or collection of "associated"-transaction) along with the date and name of both counterparties and a host of other details.