I asked some US employees about this and confirmed what I heard here - basically, you can't say you want to separate from your bags in regular operations. In irregular operations reroutes, it is OK if US initiates the change.
I got into a chicken-and-egg discussion though - in my situation, the employee would not reroute since we had checked bags and we cannot tell him to reroute because of the bag... So, we were stuck?
How does one educate those employees who are not familiar with the rule? Is there a SHARES GG topic on this? Thanks.