OK, making sure I didn't miss something. The issue with this client is they have several divisions. Most employees only belong to one division or another. However, the back office employees have to be able to send from any domain. I could have set the domains up separately, but it would have required the back office employees to have 5 different tabs open, not to mention the administrative headache.
I had created a JS script that based on the URL they logged in with it would switch the from address automatically, but it broke a few minor versions back. It still "kinda" works, meaning the from address changes, but it doesn't trigger the callback to change the signature.