Data point as of April 2018:

It took a few HUCAs before I found someone willing to do it, but I was able to change my wife's last name on an upcoming reservation. First two agents told me to bring both IDs (old and new) as well as marriage license. Third one was very happy to investigate and had it done in about 5 minutes. Used the MP self-service portal to upload other documentation to get her name changed on the MP account. This was for a domestic res ~4 months in advance.

Overall, simple process, though it seems not all agents know that it can be done.