Hi Ralph,
Interesting, the ISP
Be created a an interface for their supplied Speedtouch 716v5WL/780 router which includes a "Game&Application" preset for iChat. Someone sent me a dump of the router's config file back in April and I also discovered 4099 employed as a trigger port all by itself. With UPnP enabled and SIP bound to 5060 (the opposite of v4 firmware), just one rule on the BeBox makes iChat work.
CLI command v5 and v6 firmware:
=>service host add name="Apple iChat"
=>service host rule add name="Apple iChat" protocol=tcp portrange=5190-5190 triggerport=4099 triggerprotocol=tcp
=>service host assign name="Apple iChat" host=<enter LAN IP address> log=enabled
However here on Bulldog this CLI doesn't change a thing (no success) and since then I've been toying with the idea that port 4099 could be irrelevant to iChat but it might help firmware built to listen on the 'wrong' port, effectively bouncing connections into the right port 5190 for iChat.
5193 might suggest a routers firmware is already engaged on 5190-5192. I'm sorry I can't be more definitive but if I do discover anything useful you'll be the first to know.