You could probably use a Dropbox link. Or use a password-protected ZIP upload in-thread and PM the password. Both methods have been used for handling sensitive info in the past (and in the absence of a proper bugtracker).
But MobileRead strongly discourages uploading copyrighted books (even if they are free to download they are not free to distribute).
Even for debugging purposes (I suspect it may be because it *looks* bad).
You can, however, use @jackie_w's
ScrambleEbook utility for troubleshooting ebooks.
Presumably you should be able to duplicate the error with a Kindlegenned output from that, in which case upload the scrambled book.