No need to upgrade. EEM has been around since 12.3(4)T, and you have EEM 2.4 in your version of code, so you have a lot of power. You will still need to create a Tcl policy, though. If you upgrade to 12.4(22)T, you can do what you need with CLI alone.
Given that you're going to be doing this on a router, and if the broadcast ping works for you, run the ping command via EEM, then parse the output. If you get a response, assume there is at least one host on the LAN.
A more reliable choice is to cycle through all available LAN addresses, and send one quick ping to each:
foreach ip $ips {
set result [run_cli [list "ping ip $ip repeat 1 timeout 0"]]
if { [regexp -line {^!} $result] } {
set found_host 1
break
}
}
