If the COI remains posted it must be followed exactly.
Technically, if one wants to operate beyond the scope of the COI, it should be surrendered to the UCSG first, and reapplied for afterwards.
However, it is, or at least use to be, a common practice to remove the COI from the vessel while operating beyond the scope, and repost it afterwards. The USCG use to accept this when it was being done by responsible operators for legitimate reasons. Typically,it was done when a vessel limited to inland routes was making an offshore passage without passengers from the summer route to the winter route. That is essentially what the Bounty was doing. I would not be surprised if the USCG has tightened up on this post Bounty.
It is also possible to get temporary permits from the captain of the port for special events, or at least it use to be.
If the owner is uncooperative about this, you probably don’t want to work for him. This is just the beginning of other violations he will expect you to do. If you need the job, you’ll need to make up your own mind about what to do.