It seems like subscribing to both lease offers and expirations would
give you the ability to keep track of all the leases (well, any that
have occurred since your map connection...)
I'm a bit rusty, but can't you query the ip-mac links?
ip-mac is a standard link type, so I think that *should* work...
perhaps somebody else can demonstrate exactly how to do that. :)
Another possible solution is to create a dummy link between every single one of the ip-address that are available from DHCP, with a single dummy identifier. That way, you can just subscribe on the dummy identifier, with depth=2, and set filter to only see update on "ip-mac". But it maybe resource costly if you have a lot of IPs.