What can be done? I do not have the option of forcing plain text formatting
across the board, and I have no way of automatically generating a plain text
based signature in addition to the HTML based signature.
You might consider using eMailSignature software where you can auto
generate a plain text (and RTF)
based signature based on your HTML, and deploy across the board to
Outlook/OWA clients (in mixed environment) - without installing
anything on servers/clients as deployment is data base driven.
You're done in one hour.
Try it here:
http://www.emailsignature.eu/phpBB2/emailsignature-download-f13.html