Actually, that's where the whole affair becomes rather unpalatable as a business proposition.
Just because your modification passes the regulatory hurdles and gains approval, it doesn't mean the certifying authority is at any time underwriting your work. It's entirely possible that a civil lawsuit further down the line could find you guilty of negligence despite having received approval for your mod.
This sort of thing is exactly what killed many GA manufacturers in the last few decades. If you're interested, have a google around for some of the lawsuits held against Cessna, Lycoming et al.