Hmm. This would be a little work, but I think you could create a formula field on the Tasks table to return what the predecessor should be (via some combination of summary/lookup fields and logic), then use a webhook to set those values into an actual predecessor field.
So if s1 should always have a predecessor of p2, you could pull the RID# of the p2 Task down (from the Project to the Site), then know what the predecessor should be for s1. Make sense?
Not sure how this would work if a Project has multiple Sites, though...
