I have to agree with R1000K3 at this point.
If I understand what you've said correctly, you are giving a warranty to cover failures related to parts you sell. But the car must be under the factory BMW warranty? The general practice has been to remove mods when a person goes to the dealer with a problem, therefore the BMW warranty covers it.
I suppose one could say what if the dealer says 'no way' but that's a big what if. If you've removed the mods, then its up to the dealer to prove mods even caused the problem. Maybe I'm just splitting hairs...
Good luck with your new idea!