What you say certainly sounds like it makes sense and could be possible.
I've had a look at miniupnpd and on a first pass did not get it to work. If you want to try yourself, apart from the normal development tools, you'll need the ipdables devel package.
yum install iptables-devel
Then follow the instructions in the INSTALL file. I had to edit netfilter/iptables_init.sh and netfilter/iptables_removeall.sh to change my external interface to eth1. Really these scripts should be rewritten to use functions-automagic or read /etc/firewall to determine the external interface(s). The rest of the build seems to be OK.
The next problem is the init script which uses /lib/lsb/init-functions which are not implemented in ClearOS. This will need to be rewritten to use /etc/rc.d/init.d/functions instead. This should not be a desperately difficult job but I don't really have the set up to test the end result as, having moved P2P to transmission, I no longer use any software requiring uPNP. For this I've only been playing round in a VM.