I don’t ever say something is impossible within Power BI anymore, because someone more clever almost always comes along and proves me wrong. However, I don’t see a way to do this within native Power BI.
On the other hand, if the client is willing to use R in conjunction with Power BI, this is a simple task. Per the article below you can add any line to a scatter plot within R. Alternatively (or in addition) you can also easily calculate the regression line and the residuals, and then in Power BI conditionally format each point based on whether it is greater than or less than your threshold residual amount.
For a look at how you can calculate, plot and build measures based on R-generated residuals, check out the entry we put together for the COVID dashboard Data Challenge a few months ago:
I hope this is helpful.
– Brian