You've got no chance of ejecting the CD from a %post. The CD is most definately in use (as SEP mentions).
The RH installer usually ejects it it's self.
But as you are doing a KS, ... yea, I have the same issue (but have just been ignoring it) ;) (KS, booting from CD, installing from NFS).
Unfortunately, I don't think it can be done.
Which leaves you the option of making a boot diskette (shrinking driver modules to fit on a single floppy is a pain-in-the-arse!) and ejecting that, creating a fully networked installer (private segment with a DHCP server etc.), or ... dunno, haven't done the 'or' yet..
Sorry.
One long-haired git at your service...