I've long wanted to have lower level amateur weekend leagues but, unlike yourself, have not felt like editing schedules for a few versions now.
If it was only simple enough for the game to generate a six month schedule for a 48 game season...
So this is a feature I would find consistent use for, I think something in the set up that allows extra days off to be added, and as you referenced, setting the frequency of occurrence would be a great addition!
Just the other day I was thinking about having a feature within the schedule editor to insert days off to existing league schedules, but to do it as a part of league creation itself would be even better. Though it could be both of course.