Iknownot: just trying to add something to your post, the REDIRECT target is correct.
Yes, I believe you are correct. Apparently it is used to intercept all requests to the outside, from inside. I was thinking the other way, since I am used to seeing this with changing the ports from within.

I still think that a proxy inst necessary on that configuration and just make the thing worst. However im too lazzy to write an entire iptables configuration here
Remember, one has to consider the firewall used. If default policies are set to deny ( as they should, and not necessarily the default ) it may be more difficult to get this thing working. And the helpers ( sound, etc. ) won't be forwarded with just a NAT of port 1720. ( Again, haven't tried this in years ... but when I did it wouldn't work, thus the necessity of the h323-conntrack-nat - h323 connection tracking and NAT helper .)

As bAgZ confirmed, this is a much simpler way to set something like this up instead of patching Netfilter and the kernel, but I am still concerned with those rules.