I am not a tax specialist and in all likelihood this is how it would be taxed, but think of the logic (or maybe the lack of it in our 1040).
Say, I had 100 coins on MtGox (I didn't have any, just a theoretical number).
They are sending me back 25 and telling me that 75 were "lost" due to the business failure.
Let's say, i get the coins today, July 17.
OK, I got the "income" of 25X65 (roughly for simplification)=$1.625 mil
In addition, I also got a claim of a capital loss of 75X65=$4.875 mil. It is real, but I am not sure how to best document it in the return-a question to CPA.
I'm not a tax specialist either, and especially not on US taxes, but shouldn't the capital loss be based on what you paid for it back in the days? You never paid capital gains tax on those 75 Bitcoins, so I'd say the capital loss is based on what you paid in (say) 2014.