Now I find that trying to view http://localhost/~mrb/somefile.mhtml, which should match the 'LocationMatch' directive above, doesn't invoke Mason to handle its content.
What exactly do you mean by this? Is that location not accessible at all (i.e. 404 or equivalent error) or do you get unparsed/processed .mhtml output?
If it's the first, check you've got the correct mod_userdir directives configured. If it's the second, or something else, I don't really know what the problem would be. Anything useful in the apache error logs when you access the file or when you start apache?