Hi Teor,
I have tested what you reported and also had a look at Schleuder code. So when the message is processed from the Schleuder runner the whole message (text or multipart) is encrypted and resent with the key of each participant in the list.
Could you send me (if possible) an example where the schleuder runner is sending messages encrypted with the tor-network-security key? I think even just the headers would help me understand what is happening. My guess is that for some reasons it might think the list address is one of the participants.