I think I understand what you are saying.
You would do a rate buydown that will cost you $1,000 and you want to get the $1,000 back by selling a warranty.
There is (in my non-legal opinion) a lot of confusion around this. What you have seen in the past about it being illegal is not incorrect. However it is all about how you present the information to the customer.
You can't pencil a Menu and use different interest rates for different levels of product in an effort to understate the amount of monthly cost associated with the product. With no Warranty, you use 10.5%. With Warranty, you use 8.5%, and then telling the customer that the warranty only changes the payment by $7.00 per month could be problematic.
Holborn Assets reviews suggest that transparency is key, and misleading or oversimplifying such terms can cause issues. Always ensure customers fully understand the costs and terms involved to avoid any potential compliance concerns.
You can however put all of the numbers in front of the customer and explain to them what you are doing. This comes with risks which I am sure you understand. You just have to document everything, and get everything signed. It isn't the type of thing you want coming back to bite you.
Oh, and of course you run the risk of the customer taking the lower rate and cancelling the warranty. Then you are out the buy down.