According to CHDK Wiki CameraFeatures page, it seems that PowerShot A810 can't go beyond 15 seconds of original long exposure limit. A1300 isn't listed, but it's probably same.
So, has anyone tested one of those two models further - can they go at least 60 seconds? Some people say A810 was easy to port, so I see no reason for such limit since fastest shutter speed with CHDK is 1/100.000 seconds, and ISO can be cranked up to ridiculous 10.000.