Personally I would just use the StarSan -- the foaming agent reduces surface tension and will let it wick into the crevice nicely. I've eliminated bleach entirely from my brewery.
OTOH, I'd worry about the integrity of the carboy a) when it's full of wort and you have 80+ lbs of liquid exerting pressure on the crack and b) if your ferment blows off and any restriction increases pressure ........ <boom>
Keep the carboy in a tray or a bucket ....... well, if you have a bucket why not ferment in that ........ I might even ferment in my boil kettle if it came down to using a carboy with a crack in it. If you're out of beer, this batch isn't going to last long -- pitch and aerate well and drink it before any off flavors develop