I agree that this would be a neat feature. This is not currently a feature in osTicket at this time. I do not know if it is on the roadmap either. note: I find that most of the time when i try to do this in gmail it bounces back to me, but that is because I use the SMTP server for the mail host that the email is hosted at instead of gmail's.
I imagine that you would have to edit /include/class.email.php and then edit the file where you want to put the override setting. Example if you wanted the Department to have the override then you would probably edit the /include/class.email.php and/or /include/staff/email.inc.php and emails.inc.php. This is not something that I have tried to implement.
I would recommend that if you add this (with a setting some where like in departments, or in emails, etc.) that you do a Pull Request and try to add the feature via github so that the feature could be accepted into the project.