Assuming - even though not specifically mentioned - that bridged mode is blocked there's nothing preventing you from using your own router is there?
I use my SMC router, RouteSentry and my Nashua account together with a Telkom account and it works fine.
Would ideally like to be able to make use of the wireless capabilities of the Billion though... sn3rd tried to talk me through connecting the SMC to the Billion but I only got it half working. Need to put some more effort into it I reckon.
The jackpot would be flashing that Nashua Billion, so if anyone knows how or has a dodgy contact who will do it please let me know!