This is a disingenuous claim, because many of the comments at the linked thread indicate that it not only is a breach of the RFC requirements, but is also liabl

This is a disingenuous claim, because many of the comments at the linked thread indicate that it not only is a breach of the RFC requirements, but is also liable to malfunction, so any provider/software that permits it, is going to have trouble supporting it. I stand by what I said: it's impossible, and is simply not done [by anyone who cares about adhering to standards or interoperability.]