OK, so in the logs, I see:
ftp9lxv.mitchell.com proftpd 10.0.54.70 (172.20.165.2[172.20.165.2]): session requested from client in 'trace' class
which shows that proftpd correctly identified the client as belonging to the 'trace' class. Interestingly, though, I don't see the log messages showing mod_ifsession merging in the <IfClass> changes, or them being picked up. Hrm. I'll see if I can reproduce this locally.