I'm not sure IRS is aware of the ownership of unredeemed bonds.  Treasury could be issuing 1099's in the year of final maturity -- or, do they even have SSN's from those issued three or four decades ago?  Once IRS received the 1099, it would have the last known address for many of the owners.  
I would leave it up to the heirs to decide whether they wanted to pay tax on income that should have been reported long ago.  I don't think it's any different from dividends paid long ago and turned over to the state unclaimed-property administrator.  Not all of those were reported as income, either.