Granted, we're in somewhat uncharted territory here, but I really don't see what the problem is. For 'merkans, probably the closest model is a corporate spinout - like when HP (HPQ) split into HP Inc and HPE. On the day of the split, the cost bases were adjusted (or determined). The value of HP Inc stock plus HPE stock at t=0+ was equal to the value of HPQ at t=0-. No taxable event, just an accounting adjustment to the cost basis for when you actually do sell.

Of course, when you do sell, that is a taxable event. Did you expect otherwise? Of course the IRS is gonna get a share of your windfall. But using this model*, the tax would be owed upon the gain, which would be based upon the cost basis determined at the fork -- not on the entire value of what you are selling.

*Again, this is likely uncharted territory. But this is a perfectly fine model. I can't imagine anything closer. Can you?