This sounds like the best solution to me, I don't think that's really outrageous to have you configure the network when connected through USB. Many phones will do wifi-tethering so you could actually have the mod connect directly to it. I wasn't trying to suggest wifi is easy, but I just think its going to be the best way.
Wifi will definitely have the most problems with customers with poor network setups, routers with bugs, etc. I've heard thats the #1 cause of customer service calls at apple, at my work we've seen much the same. There's just no getting around that, you just have to be ready to help people.
Do you mean captive portals, like at libraries or universities where you have to agree to terms of service when you connect? That will be difficult. I don't know if its even possible without a browser... I guess if somehow you could forward it through the USB connection when the user is setting up the mod... Anyhow, I imagine you still have a lot to do before you get to that.