We use a certain username format for our staff users (_). To make sure “normal” users don’t look like staff in any way I’ve blocked certain characters under reserved usernames
Problem is, as an admin I can’t even circumvent the filter to change a username. When a new staff member joins I have to remove the filter, change their username and quickly re-enable the filter.
As an admin I’d like to not be blocked by that setting.
Yeah, that’s annoying, but I’m often annoyed that the tag requirements that I want enforced are not enforced for admins, so it could be just as annoying to change it to give admins license to break the rules that they have tried to impose on themselves. 
And from the Jay-always-treats-feature-as-support department, you can also use Automatically add group flair to members’ avatars which is much less subtle, so another way to solve your actual problem is to let the flair do the job of making it clear who is staff.
Maybe you have too many admins?
I use flairs too, but somehow there are always users who refuse to see, so I’m looking for everything I can get to help them.
You mention staff users; is this for a company with a recognisable logo?
If so, you could always take a leaf out of Blizzard’s book.
Another way to make staff replies stand out is by adding a background color to their posts as per
Scroll down to Post #9 explaining where to make the change in the css.
Thanks for your thoughts, everyone.