I use the following trick:
iptables -t nat -A OUTPUT -p udp --dport 21027 -j DNAT --to-destination 127.0.0.1:21026
i.e. as another instance should moved to listen 21026 port, and when UDP-broadcast packet is sent to the default port (21027) it is also copied the port 21026, and voala!.
The trick works on linux. No idea about other OS’es.