Ah- yes, it should work. Just be sure to:
1. Create a new theme folder (don’t try to override by putting the view file in the default theme)
2. Create a folder structure that mimics the ‘view’ folder- in this case, created a _shared folder and then put a copy of header.php in it. Edit header.php as needed.
3. Be sure to switch to your new theme! Whee- I forgot to do that my first test and was like ‘why the heck doesn’t this work’. Heh.
And with a custom theme, you will keep a lookout for changes to view files- they should be posted in the update notes each release. Not that the header is going to change very often, but if it does, you’ll want to update your custom theme.
You COULD do it w/an accessory showing in all cp pages, but I’d go custom cp theme route myself. It’s easy to maintain since you only need to put the files you want to override in there.
That do the trick?