Type sysctl -a | grep icmp to see all kernel variables related to ICMP. You have icmplim which limit the number of ICMP packets by second and log_redirect, drop_redirect ...
In the same way, look in the kernel configuration file (in /usr/src/sys/i386/conf), you'll find ICMP options.
In IPFW, you'll only find parameters to drop or not ICMP packets.
I think DUMMYNET is only for TCP not for ICMP.
Hope it helps