Give this a shot...
Router
logging 10.1.1.100
PIX
access-list nonat permit ip host 10.1.1.100 host 5.5.5.5
nat (inside) 0 access-list nonat
access-list outside_access_in permit udp host 5.5.5.5 host 10.1.1.100 eq syslog
access-group outside_access_in in interface outside
Hope it helps. Please rate helpful posts.
edit: Oh and one more thing, the outside router will need a route to the syslog server. Something like...
ip route 10.1.1.100 255.255.255.255 5.5.5.6