This method works for me and i'm sure it will work for everyone as well. I only have a telkom account so i can't confirm this will work with anything else. If it is after the 1st of the month then you can stop reading and come back when its the end of the month.
I set my Mega 105WR router to PPPoE (10.0.0.2 -> advanced -> WAN -> 'quickstart' -> Type). Then I enter the PPP Settings on the router to my telkom account. Now essentially this account is always connected. Anyone who connects to the router will have access to this account. If i wanted to use another account i would just 'dial-up' to the router with the other account information and it should override the router's account (i think - someone correct me if i'm wrong).
Now that you have the router set up simply, by the end of the month, be capped. Leave this account online when it goes from the end of the month to the beginning of the next month. Since you have not disconnected telkom still registers you as being on local cap. It will start using the next months local allowance. Now if you check
https://secure.telkomsa.net/titracker it will show you that you have at least 1 active local only session.
If you want to access local off that connection simply create a 'dial-up' connection but with your telkom account information. Even though you already connected through your router to this account, when you dial up it creates a new connection and thus creates an international connect. Which you should be able to see on the titracker site.
Now as long as you aren't disconnected you should be able to do this for the month. I did it for about 6 days last month, but then my router disconnected.
Good luck for next month