Makes total sense, and the way I was thinking of adapting it wouldn’t work because of the “two red days in a row” problem. Thanks for clarifying all this!
Does beelint recognize binary/nonzero goals? That is, if I have a binary goal that needs to be done twice a week, and have beelint do it weekends-only, will it notify me on Saturday or think I can do it twice on Sunday?
Looks like this is not true, actually 
Yes, it has limited support for binary goals. Specifically, there’s a config option always_valid_if_data_today which makes it so the goal cannot have a lint error if it already has a data point for the day.
For example, suppose you have a weekends-only goal that’s set to derail on Thursday. Today is Saturday so you can and should work on it. Beelint will complain about the goal until you put in a data point. If that shifts the eep day to Friday, then on Sunday you’ll get a new lint error when you wake up. If it shifted the eep day to Saturday you wouldn’t get a lint error (you can just wait until next Saturday to work on it).
The thing that it doesn’t do is detect a hopeless situation. For example, if the goal is configured as 0/1 per day and you set the rate to 3 per week, it’s impossible because there’s only two weekend days per week. Beelint won’t notice this and will allow you to derail the goal.
It is also very literal about “a data point” regardless of the value or aggday effects. An improvement would be to support a setting like “3 is the maximum value for each day” but that is not implemented currently.
[Warning: I have not read every word above so this may already be fully addressed].
I also want this feature, and I want it for a different reason than @apolyton. I don’t want to be in a situation during the work week where I have to choose between derailing on a side project goal and deprioritizing my actual job. I know that if I planned ahead, I could avoid that situation, but the value of beeminder for me is precisely that I don’t always plan ahead well.
I want the eep-day situations: the “Do I want to avoid this thing more than I want to keep $30?” But for some goals, I only want them on weekends.
I’m going to try Beescheduler right now.