Sorry, maybe we’re just miscommunicating. 
To take a step back, the fundamental things that I want from Beeminder is a guarantee that “When you set your Road Dials, you’ll be forced to do those things at those rates” and “If you have no eep-ing goals, you don’t need to do anything today.”. Beeminder bugs which negate these guarantees are the ones which frustrate me the most.
For example, you can actually exceed the rate you specify on a Do Less goal because Beeminder doesn’t actually enforce the limit it displays in the UI. So if you think “Ah, my Do Less goal says my limit today is X so if I do more than X I’ll derail” you are mistaken. (Details at Why don't I derail? - #2 by drtall ).
There’s also failures of the “bright line” principle, for example What do the colors actually mean? - #2 by drtall . This makes it harder to reason about whether you need to do more for your goals and therefore makes it more likely you derail for non-legit reasons.
Whenever you derail for a non-legit reason or fail to derail when you should, Beeminder has failed as a platform. I say this because you’re simultaneously not doing The Things you want to do and you’re also not being stung.
Now to come back to the topic at hand, a goal which can only be performed on specific days of the week is prone to non-legit derails. If you can only do it on Saturdays and you derail on a Saturday, that’s a sting and you deserve it. But if it derails on a Tuesday, it’s not legit. Sure you don’t lose money for the Tuesday derail, but you’re also not accomplishing the goal you set out to do.
So you have to make a choice: 1) be okay with Beeminder not actually forcing you to do The Things (in which case why am I using it at all?) or 2) do something to ensure that eep days only fall on legit days where you can actually do The Thing.
Suggestions of the form “You should look ahead in your calendar” miss the fact that the entire point of Beeminder is that it tells you what you need to do today. So if Beeminder can’t correctly determine that you need to do The Things today, then Beeminder is less useful than it could be.
My suggestion of automating this “beelint” goal such that it eeps whenever you need to do future work for a non-eeping goal is satisfactory because it maintains the principle that “If I have no eeps, I don’t need to do anything today”.