As a bona fide repairer, can you not obtain this information from the manufacturer(s) concerned?
If not, might you have a case under any Restrictive Practices legislation in your territory? Are you a member of any Trade Association which might be able to assist you in pursuing such a claim?

Even though you are totally above-board yourself, you might find that this information is available on some of the less scrupulous sites in the shadier corners of the Internet! (well, that's the way it seems to me, having sought to find some technical information on mobile phones!)