The easy way out: The buck stops here

The easiest solution could very well be to let all ICMP traffic from your local net through and let probes from elsewhere stop at your gateway:

pass inet proto icmp from $localnet 
pass inet proto icmp to $ext_if 

Stopping probes at the gateway might be an attractive option anyway, but let us have a look at a few other options which will show you some of PF's flexibility.