template (name="my_fmt" type="string" string="<%PRI%>%TIMESTAMP% %HOSTNAME% %syslogtag% %msg%n")
if ( ... ) then {
    action(type="ompipe" Pipe="/tmp/SITENAME/pipe" Template="my_fmt")
    stop
}

set ACTUAL_SITE = "foo";

template (name="my_fmt" type="string" string="<%PRI%>%TIMESTAMP% %HOSTNAME% %syslogtag% %msg%n")
if ( ... ) then {
    action(type="ompipe" Pipe="/tmp/$ACTUAL_SITE/pipe" Template="my_fmt")
    stop
}