There are non-Mac utilities to read .dmg files. The NDA and the $99 fee are bigger obstacles to wider uninvited beta adoption, I'd assume, but intentional. This may or may not be a good thing: more testers isn't always better for bug report quality, and for a hardware company like Apple, it'd be bad business to encourage widespread adoption of beta firmware, without either support for devices running beta software (unlikely), nor support for uninstalling beta software. It's probably not even legal in all jurisdictions.