Yes with a Canon wireless Speedlite like the High-Power Flash HF-DC2 or the DC1 which comes with a mounting bracket that attaches to the tripod mount on the camera, here is the link for it:
Yes the 430EX II specs says it is compatible with all Powershot G series cameras but the 600 Ex Rt does not...you might want to call Canon's support people up to confirm that on the 600EX RT if you prefer that model.
