The best definition I know of for "firewall" is "Network Policy Enforcement Device". So if you engineer an app to bypass typical firewalls, what you've created is, by definition, a "Network Policy VIOLATION Device". So the end users you're trying to help go from just not being able to use an unauthorized application, to potentially being FIRED for trying. User friendly? Hardly.
Look, guys: If you build your nifty thingamabob assuming that network security is your users' enemy, guess what? IT WILL BE.
Play nice. Use your own ports, register and document them. I routinely Google on "product name" and "firewall" to learn what I have to do to allow my users to use said product, and make the appropriate adjustments, usually within 24 hours of the first request from a user that gets approved.
But pull a stunt like Yahoo, and I have to start blocking addresses and checking the status of funding for an SSL proxy and possibly making it a bit hard for our users to get to some approved destinations while figuring out how to block your crap. Result is that I'm not happy, and neither are my users, and so when it reaches someone who can approve the use of your app -- or NOT! -- on our network, my recommendation is going to be "No, we can't trust them" and odds are that the blocks will be made permanent.
And it will be your own fault.