Don't think it's available as a nicely packaged 'web service' for the 'lazy' programmers
If they do offer it, it might not be 100% up-to-date though, because the cap is usually not monitored real-time. Usually you can get stats up to the last session, and if you're lucky the current session is also guesstimated periodically.
Don't know about SAIX offering a nice API, I think the SASA utility just parsed the ADSL stats page. WebAfrica has (had?) an API of sorts for getting stats for their accounts.