Well, replying to myself again. I got a script that works. I tried adapting it to invoke on Command-R (sending a command-R to mail as the first action, but that blew up badly). Still, this works, using a named clipboard to store the file contents, then using a “type characters return-dash-dash-space-return” and then pasting the named clipboard.
tell application "System Events"
tell application process "Mail"
set frontmost to true
set value of attribute "AXFocused" of UI element 1 of scroll area 1 of front window to true
tell application "System Events" to key code 125 using command down
The script tends to fail if Mail is in full screen mode, but poking around it seems that FSM is the enemy of all scripting, so I don’t think this is something KMP could fix.