As promised, I have completed the preference related commands and pushed it as revision 60 .
The preferences form a new scope, along with user, domains and lists.
There are 2 commands associated
./mmclient show preference [scope] [scope_args] [key]
That is, to view a preference of a list member
./mmclient show preference member --email email@example.com --list firstname.lastname@example.org receive_own_postings
Similar to the above command
./mmclient show preference [scope] [scope_args] [key] [value]
./mmclient show preference member --email email@example.com --list firstname.lastname@example.org receive_own_postings true
The supported preference scopes are
global  user --email address --email member --email --list
Also, the command scope resolution code in cmdparser.py, which was repeated once per scope, has been replaced with a better and faster approach.
 Upon updating/editing the global preferences, a
HTTP Error 405 : Method not allowed is raised. I tried as sudo but still
no good. Is that a mailman bug or a design decision?