You should be able to re-enable plaintext auth with a custom template
for /etc/atalk/config -- if you look at that file, you'll see the UAM list.
That part of /etc/atalk/config is templated in
/etc/e-smith/templates/etc/atalk/config/20AfpdUAMLIST, so something
like this (untested) ought to work:
mkdir -p /etc/e-smith/templates-custom/etc/atalk/config
cd /etc/e-smith/templates-custom/etc/atalk/config
cp /etc/e-smith/templates/etc/atalk/config/20AfpdUAMLIST .
## note the space followed by a period at the end of the previous line! ##
perl -i -pe 's/so"/so,uams_clrtxt.so"/' 20AfpdUAMLIST
/sbin/e-smith/expand-template /etc/atalk/config
(check the file /etc/atalk/config by hand here to see that it looks right)
/etc/init.d/atalk restart
Since atalk startup is backgrounded (because zip_getnetinfo takes a long
time on a network without appletalk routers), watch /var/log/messages
for errors, and for the message
Feb 17 00:22:37 hostname afpd[24141]: uam: "Cleartxt Passwrd" available
indicating success.
Keep in mind that this means that users' passwords will be traversing
the network in the clear, readable by anyone watching traffic on the
network -- which happens to be one of the reasons we removed support
for cleartext passwords in 5.6.

Cheers,
--Rich