Unfortunately, I just worked though that link and it does not help. I've tried both self signed and publicly signed certificates and none of them will show on connections to port 7831. They do work for the regular captive portal. (not going though the explicit proxy) I even just tried the factory CA certificate as shown in that example, but it did not work either.
It is also interesting to note in the last screen shot of that link it looks like they are ignoring the certificate error too. Notice the yellow warning triangle in the address bar. Hopefully there is a way to change the SSL certificate presented on port 7831.
Thanks for your assistance!