A separate log file would be an overkill here. To me it makes sense to keep it within the ```y2log``` because during reading the log you can see when the hooks are called in context of the other logged stuff as this happens during the whole installation workflow. Every hook (if not used) adds only 3 lines to the log file and those are not longer than 50 characters. 
I would not be afraid of this hook summary to be adding any excessive content as the number of used hooks will be very rarely higher than a dozen.