FreeBSD: redirect some UDP traffic from one port to another

Question

I have some specialized DNS servers running on FreeBSD 11. A few people keep hammering on them and I would like to give them different answers. So I'd like to divert UDP traffic from those hosts from port 53 to port 5300 where a different server is listening. I tried ipfw fwd, which gets the traffic to the port 5300 server, but the answers get lost because they go out with port 5300 rather than 53. So it looks like ipfw divert and natd should be able to get the requests to port 5300 and the responses rewritten as though from port 53, but I haven't been able to figure out the right set of commands and options. Examples would be very helpful. (The DNS servers are custom stuff utterly unlike BIND, so I can't just do split horizon.)