The Outlook signatures are stored in the %APPDATA%\Microsoft\Signatures\ folder in text, RTF and HTML formats. In case of multiple signatures, the default can be found through Word automation. Tested in Outlook 2003 and later.

The Advanced Security for Outlook

from http://www.mapilab.com/outlook/security/ is free Outlook add-in that allows end-user to take control over Outlook security alerts.

The Outlook model doesn't provide access to the Label property, but it can be retrieved/changed via CDO 1.21.
Requires Outlook 2002 or later. Tested with Outlook 2003 only.
Based on http://www.outlookcode.com/codedetail.aspx?id=139

Code is based on Building Applications with Microsoft Outlook Version 2002