In order to facilitate netwide signoff (or query) without flooding the
net with messages, why not try the following:
Have each backbone LISTSERV keep a list of subscribers to lists anywhere
at nodes "under their jurisdiction" (i.e. for which they are the nearest
listserv.  They would need to be primed initially, and then for every
subscription modification a record would be sent by whatever listserv
maintained the list to the closest backbone.  They might be batched (say
once a day).  That way, one would only need to ask the closest backbone
to find out what subscriptions you had anywhere.
                 Victor Miller -- IBM Research