Yes, as I understand the rules, he can be nominated as the home marquee player.
I suppose the question is whether he is earning more than George Ford or Dan Cole, which are 2 positions that traditionally are the higher earners in a club.
Dan Cole's best is behind him now, but that rarely affects salaries.
George Ford was first choice England fly half so is likely on a good salary too.
I suspect neither are on £600k though. It's only in the last couple of years that salaries like that have been considered