You will have to combine AXL and RisPort to get to the full picture. You do need AXL anyway in order to be able to limit your query to 200 (now 1000 in the 10.x train iirc) - and then you create your query, specifying the 200 devices, and then you compare the result you get from risport with all the devices you asked for. Any device that is not part of the list, is not registered. I've been generating my registered/unregistered list like that since CUCM4. I haven't tried against 10.x yet, but on 9.1 (my last big project), things still worked as expected.
I guess that it's a matter of how long this data is being kept in the cache.. devices that are registered, are obviously known. Then there's those that recently unregistered - they should show up as well. But long time offline devices won't be returned no matter the filter.