There is no such event that I am aware of. Maybe you can explain your need for such an event better (i.e. your workflow) and I can make a recommendation?
WAB/LaunchpadTheme is responsive and on smaller devices we'd like the logo to be a link to the main site.
Short term solution was to add "Home" to the links array in config.json since that's always visible. Using JS we could wrap an A tag around the logo image, but I quickly realized that it takes a few seconds for those DOM elements to appear on the page.
Enhancing the theme header widget will definitely be the better, long-term option for us.
You are correct that modifying the launchpad themes header Widget.js is the best solution. But if you really need a short term solution than you add some code listen from the widget creation event topic. topic.subscribe("widgetCreated") and see if it is the Header widget and if so then do your dom manipulation.
Interesting... I've yet to create a custom widget (or do any dojo/dijit development) so dojo/topic is new to me - thanks for the additional (and cleaner) workaround.