Hmm. I figured that there was a hook that contained the private message ID somewhere, so you could know exactly what to query. But I haven't looked at the hooks or dumped any arrays to see what is available.
Once you have the PM ID#, I was thinking just to do a SELECT lookup, retrieve the date/time, then make that available as a $variable that can be used within the template.
For the moment, I'd just like the email bug fixed