I spoke by telephone to Olena Ruth, Esq., who appeared in my blogpost “Assigned Counsel?”, 1/6/16. Ms. Ruth assures me that the CO event described in said blogpost is purely voluntary, pro bono, and ad hoc. There is no formal system, and certainly no assignment of counsel in Tax Court.
As for Ch J Michael B. (“Iron Mike”) Thornton’s delegation to IRS counsel, I have no more to say than I have already said (see my blogpost “Assigned Counsel? – Part Deux,” 1/28/16).