Transport Rule to restrict Character Limit to Specific Domain.

Hello

What we want to do is restrict the content of emails sent to our Text\SMS delivery Domain to stop one email being coverted into 3,4,5 or more texts.

The Character Limit for a Single SMS is 160, I cant see any way of doing this using a Transport Rule ("Where the Recipient Domain is SMSDomain.com), or DLP.

Has anyone written a custom Transport Agent or Rule that has achieved this or similar?

We have looked at using the Disclaimer to Prepend or Append an **EOM** string, but we really want to configure a hard Character Limit.

Thanks