There is no way to do it other than set it at the device per scan.
We have a 7845 with Scan to email configured and working using pre-loaded addresses (in address book and set as Favorites).
The problem is the default subject is alway the same, and pretty non-sensical. Our process requires we scan copies of documents to standard email boxes, but the people getting them want the subject to reflect what it is - like Travel-form, or Purchase Card receipt.
Nothing very custom or specific - just 2 general subjects.
You can use the panel to change the subject when you press the 'email' button - but frankly the touch screen keyboard is yucky and we want something consistent. People will not make it the same, it will vary.
What I'd like is to use some type of default - Workflow scanning might work but workflow scanning does not scan to email (SMTP) that I can find.
If we could put the subject in the email address in the address book - that would work.
Right now, it is manual, tediuos and error prone to type in the "subject" on the screen.