Yeah, I'm not sure of a better way to solve it.  The only thing I would consider changing is dropping two files into place - one clearly marked as an installer provider file, then another (with a later filename) with comments allowing the user to override them.

What I definitely didn't want to do was write the admin password into /etc - partly because it shouldn't be readable to non-root users, and partly because a 0600 mode file in /etc might throw up errors for non-root users.