Crossbow is AMAZING. It can do some many things really simply. What you want to look at is the usage of the flowadm command.
What you are looking to do is just a one line command that will look very much like the following :
flowadm add-flow -l bge0 -a transport=UDP -p maxbw=100M, priority=low limit-udp-1
Obviously, you want to replace the appropriate information.
Here is another example that will most likely suit you better.
flowadm add-flow -l xnf0 -a remote_ip=126.96.36.199,remote_port=43833 flow-tcp-app
flowadm set-flowprop -p maxbw=8M flow-tcp-app
That's all you've gotta do. You may be looking at local_ip instead of remote_ip but that doesn't change much. No needs for zones if you don't need them. Nice, isn't it ?