I doubt a Canon ring flash would have sufficient power to act as a fill light. Rings are associated with macro distances and have a GN that is appropriate (even if using a small aperture).
I have seen some close up portraits taken using ring flashes but with a GN of 14 or 24, depending on model, it's quite low powered and I am guessing they are used because of teh catchlights they create being a bit "different".
Not sure about other manufacturers though as I dont know their GN.
Matt