If it only trusts the certs from distro maintainers then surely it will be distributed as part of the normal package sources. If you need to add a cert, then the problem is exactly the same as adding a repository to the package manager; if the delivery mechanism of the instructions is compromised you're hosed.

Powershell will accept codesigning certs that are signed by verisign, so the workaround for an attacker who has already compromised a web site is to modify and re-sign the script with a certificate that can be obtained for $60.