Having thought about this, I don't think I want to rely on external websites to get IP address information. At the moment, DSLstats is self-contained and doesn't rely on anything external (apart from my own website). If the information isn't available from the modem/router, then my second line of approach will be to see if I can use my website to return the information.
As Stuart said, the custom command interface will provide a means of getting any additional information from the modem/router, although I suppose that if the device is in bridged mode there won't be any way to get the external IP address from it.