BTW:A far simpler set up is simply dispatching to FS boxes. Then for maintenance, you pause calls (fsctl pause) or tell FS to shut down when calls are finished (fsctl asap, as per mod_commands#shutdown).
Are you looking into track-calls for FS crashes, or for maintenance to move calls around..? This satisfies the second just fine..
-Avi
p.s. mod_ha_cluster in development should handle this beautifully... some info on mod_ha_cluster