Seems to me like a good time to require that the employees use any accumulated vacation days first so that if/when they return they aren’t off any more for the rest of the year.
Also seems to me like a whole lot of Federal employees owe the government for unpaid taxes. Anyone who does owe should have their back wages sent to the IRS to pay off that bill first. Then if there is any money left over it could go to the employees.
No one - not even Federal employees - deserve something for nothing, particularly when they are using my tax dollars to pay for it.
I'm sure the catch-up check will be handled like any other payroll check and taxes withheld.