It's fantastically kind of you to put in all this work for me - I really appreciate it. And you've cracked it! The above code - substituting 'ubermenu' for 'mytemplate' in line 1 ;) - works just as intended.

From what I found, the "parse_templates" hook location is the only one that will work for the "header" template, but at that hook, the notifications data is not available yet. So, I had to find a hook location that is global, and inject data into the "header template another way. :)