Hi John,
I, too, was more annoyed than inconvenienced by the inability of disabling the auto validation for the XML file extension. In my case, I do a lot of macro-driven examination/update of files in a fileman list. Whenever an XML file was in the list, the resulting validation error dialog would interfere with the macro's behavior. So, not really depending at all upon the benefits of auto validation, I searched for a way to disable the feature.
I managed to turn off auto validation of the XML file extension by setting it up to refer to another extension, HTML, for which I had previously unchecked "Auto validate on open" giving me the expected behavior (no validation on open).
However, in order to get the "refer to" button for XML to be enabled, I had to first change any file extension that was referring to XML to refer to some other extension. In my case, I changed all these extensions to refer to HTML instead of XML. Once the "refer to" button for the XML extension was enabled, I was able to set XML to refer to HTML. At this point all XML extension files stopped being validated on open.
I'm not really certain of what I've given up in terms of auto-completion, ... etc. by treating all these files as HTML-equivalents. The main feature of the editor that I've come to depend on is syntax highlighting, and HTML is a reasonable equivalent to XML in this respect. Someone else, though, may not be willing to sacrifice pure XML extension support in the editor as I have.