Why not just move the syslog messgae up to a higher syslog level, then only send that level off to the syslog server?
You can use the "logging message syslog_id level " command to change its level up to a higher level, then use the standard "logging trap " to send them to your syslog server.
The syslog_id of the message is the first part of the message as such:
%PIX|ASA-1-105005: (Primary) Lost Failover communications with mate on
interface interface_name.
