Load balancing is tricky and tend to require quite expensive routers due to the fact that you tend to need multiple WAN ports.
In the case of doing it via Wi-Fi, I really don't know of any commercial solutions that have a means of letting you access an internet connection that has a log-in page.
I'm afraid I'm not familiar with how it works in your case, as I haven't tried the service, so it's hard to say, but if it's anything like most Wi-Fi services, I think you're out of luck regardless of what company you go with.