I have been looking at this a lot too. I want to switch languages with a querystring and read it with a $_GET, but it seems to ignore it. I looked at the code behind the Captive Portal, but can't find where it ignores it, which is rather odd because it reads the redirurl value.
I have tried to hard code test values in the inc files, but my pages don't seem to read them. Perhaps it would be an idea to create a test PHP page to see if this can read them, and therefore get an idea where these values are being dropped.