"An operator may impose additional restrictions i.e. only training/ check captains allowed to supervise ICUS pilots etc. However, this is not required by the regs."
Wouldn't 5.40 (f) disagree with this statement? In my experience in 217 organisations 5.40 (f) has always been expanded to show a training requirement for "appointed pilots" if they do not hold any Training & Checking approvals.
Aquaplaner's statement indicates that anyone can act as an ICUS Captain, which is not strictly correct, as they must be appointed, and the appoint process is tied to the Training 7 Checking manual.
In most cases their must be a need for the person to undertake ICUS, ie command upgrade. Its not intended for FO's to claim all their flying as ICUS on the handling leg.