But before an operator allowed you to fly as ICUS, as well as the command endorsement and approval from the operator, I believe you would also need
to be recently proficiency "checked" as command (which is different to a co-pilot proficieny check).
Also to fly ICUS if you have not been checked to line as a captain, wouldn't then you only be able to be fly ICUS with a "training / Check and training captain" only ? The same way as when doing training for command and building up ICUS time.
As far as I am aware, to fly command, you need to be checked to line as command or flying ICUS with an "approved training captain".