Roundup should also work well as a user helpdesk
application.
Therefor it competes with request tracker. You should
take a look
at request tracker which work quite well.

My suggestion is to add a scheme which will allow requests
to be made by anonymous users, but record the email
addresses
as requestors. Thus it should be recorded when a mail
gets send to the 
requestor email addess.

Additionally it would be good if normal email addresses
could be added to the nosy list and kept. So the
requestor or "client" property would be use the same
Class as the nosy list which can hold a list of users
or email addesses.