That's a pretty bad flaw! thanks for testing, although most applications will let you choose which port (or randomise) for UPNP that would be a temporary work around. Devices like the XBox presumably are hard wired for one port?
To update i've manageed to build and run miniupnpd here
I had to rehash the init script a lot to get it to work, but it seems to be working very well!
Due to it's design it creates another iptables table in both the filter and nat tables to manage the UPNP connections, so it needs to have it's rules added to the custom firewall so they are not lost after a firewall restart
I'll post more in due course