I found a workable solution. iTunes exposes a "once" attribute to the AppleScript "play" command. I put this script in my Scripts menu:
Code:
tell application "iTunes"
	set sel to get the selection
	repeat with aTrack in sel
		play aTrack with once
		exit repeat
	end repeat
end tell
The "exit repeat" is there because it seems to me that a script designed to play a track once should play only one track. I haven't tested removing it. (If iTunes waits for the track to finish playing before returning to the script, it may time out. If it doesn't wait, the first track may not have time to finish before the second track stomps on it. A more complex script could address either concern, once I know which concern needs to be addressed, but at this point I don't care.)