The device table only stores some properties, the rest (including custom server properties) are in primitive_bl_value table. The relationship between the two tables is not obvious from first look; there's probably another table that maps associations, probably for bl_value_id values. Either way, what Bill suggested is much cleaner, easier, and most importantly - supported:
blcli_execute Server getFullyResolvedPropertyValue [server] HOST
blcli_storeenv Host
blcli_execute Server getFullyResolvedPropertyValue [server] FQ_HOST
blcli_storeenv FQ_Host