Okay, I won't say I've sorted out the issue, but I finally managed to get this working.
I can't pinpoint it because I did several things, including rebooting the router and recreating the port forwarding table, turning on and off my firewall in OS X and rebooting my machine. However, none of those seemed to work on their own.
What did finally work was setting PulpTunes port back to the 15000 default and then setting my router to use a different external vs internal port.
For example: the router takes incoming connections on port 4**** (the external port I set) and forwards them to my IP on port 15000 internally. To be clear, this means that your port forwarding should look like this:
Protocol: Both (TDP/UDP)
Source Address: [Blank; aka ANY]
Ext Port: 4****
Int Port: 15000
Int Address: 1**.**.**.** [my static LAN IP]
Description: PulpTunes (but whatever)
Then connecting on my WAN's IP @ http://9*.***.***.***:4**** I was able to see the server. (Finally!)
The program is still warning me at startup that it can't accept external connections, however, I tested it with my phone over 3G (i.e. outside my LAN) and was able to connect and login and see my playlists.
Could this be a bug in the way the program jives with OS X or Tomato? Whatever it is, I hope this helps others having the same issue.
I'm using Leopard which doesn't require explicit ports forwarded on the computer's OS, but people using older versions of OS X might find this helpful (found it while searching for answers today):
http://homepage.mac.com/car1son/static_port_fwd_intro.htmlThanks for trying to help Alejandro, and hopefully this helps someone else.
~G.