After trying for a day to fix my Lion-broken scripts, I finally gave up and asked folks at Apple’s discussion forum about my problem, and the very first response by Pierre L. nailed it:
Try replacing this one line:
set content to thisMessage’s source
with these two lines:
set theSource to thisMessage’s source
set content to theSource
That worked like a champ.
[Update, 31 Oct 2011]: These scripts had an issue with background sending, as the accumulated emails would clog up. I got a great suggestion for a fix on the AppleScript forums at apple.com, and now things seem to be working. I’ve replaced the scripts above with the improved copies.