Redirect Based On IP within Payload Of UDP Packet

This is actually pretty straight forward, but 1) you have to know how the data is presented in the syslog UDP payload, and 2) you have to string parse it from that payload. Here's an example:

when CLIENT_DATA {
    set src [findstr [UDP::payload] "src=" 4 ","]
    if { $src equals "172.16.99.210" } {
        pool a_pool
    }
}

where in this example the UDP payload had something like the following in it:

Request from src=172.16.99.210, to dst=10.10.10.12

Again, just an example. You'd use the UDP::payload command to expose the UDP data in the CLIENT_DATA event, and then use whatever string parsing function you need to extract the data.