I assume the anchor has a base that will be in ceiling slope, but the anchor itself will always be vertical. Maybe post a pic if it is different and someone smarter than me can help better.
I don't have good ideas how to accomplish that. But I would just not model the anchor to begin with. That seems to be too much detail to be in the model. If I wanted to show how that anchor should look like, I'd make a 2D detail. But I would think specifying that anchor (that comes from fixture manufacturer and may vary based on ceiling type) should be enough to have the contractor to do it right. I just specify the fixture and height, type of ceiling and let the contractor deal with what length cable they need and what anchors. Look at the fixture spec sheet for a start. If you model all the fasteners, you also have to schedule them and so on.
I don't recall the circumstances. But in a past project I had vaulted ceiling and pendant fixtures and was able to hang the fixtures correctly. I'm not sure if I did what is described here. But that would be a start.
If you need to model the rope or rod, you could add that to the fixture and just "adhere" to the ceiling without anchor. Adjust the length to be correct. How smart and complex you want the family to be depends on how often you come across that situation.
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