I am wondering why this import is not happening automatically at install time. There must be good reasons for that?
Anaconda doesn't actually carry out gpg checks... I think it had that added during the fedora 18/19 rewrite so EL7 might cover that but certain EL5 and EL6 won't have that ...
Since it doesn't do gpg checks I guess the rpm keys are never added as a result - as you say %post would resolve that ...
To maintain my systems I use Spacewalk and the kickstarts that are generated as part of that include importing of GPG keys listed in the spacewalk configuration...