I think I know the answer to this .... it can't be done.
Yes, that is the short answer. The longer one is: a forum member has done experiments with FPS several years ago. Some proof-of-concept code may exist, but it might be missing important parts. No-one else has been able to repeat the experiment on a different camera.
Or is it a hardware limitation?
Hard to tell. AFAIK the only DIGIC 4 or 5 Powershot camera with 30fps full HD video is the SX1. Cams with newer generation DIGICs (4+, 6) do support full HD video with more than 24 FPS.