What mechanisms need to be in place to keep it sane? ��A 'whowas' server for eg
Not at all, the responsibility for what happens with the space is entirely with the lessor. There's no reason for the public to ever know who a lessee is.
I am almost positive that there will be Legislative barriers to not keeping a record; if not now then at some point in the future. Given that even the new Syslog Spec requires a centralized DB of 'EnterpriseID' 'MUST' be included in a syslog entry; for legislative purposes - Big brother would not be happy to see leased IP's being officially moved around without some oversight.
Not that I am disagreeing with you, but just pointing out that it may be better to design a policy from the on-set with that in mind.