Hi Nathan,

One way that comes to mind (perhaps the only way) is to use a 'before' business rule on the email (sys_email) table.

Table: Email (sys_email)
When: Before
Insert/Update: True
Condition: current.type.changesTo('sent')

So essentially you are intercepting the email before it is sent from the instance and amending the body with your signature / disclaimer.

The code for the business rule could look like this:

var mybody = current.body_text + "\n";
mybody += "Enter your signature here. If required, you can use HTML tags for formatting";
current.body_text = mybody;