Regarding the boot from CD, the problem is most likely caused by a mismatch between the software on the CD and that in rootvg, which you are mounting. You can try and circumvent this by choosing "Access this volume group and start a shell before mounting the file systems" instead of "Access this Volume Group and start a shell". This way will be running completely from the CD. If this works, you'll have to mount the root filesystem (/dev/hd4) manually (say to /mnt), and the "passwd" command won't work as expected, so you will have to edit the /mnt/etc/security/passwd file manually. Set the password stanza for root to "KsXcTr5uZPZt6" (without the quotes), which is an encrypted version of "p@ssw0rd" (again without the quotes). Unmount & reboot, and you should be able to log in with this temporary password.
Regarding the mksysb, I'm not sure what you mean by a "tape disk", or whether your mksysb is currently on tape / CD / file on disk. If you have access to another AIX server, you can restore the /etc/security/passwd file from the mksysb to a temporary file, as follows:
restore -xqvf your_mksysb_device_or_filename ./etc/security/passwd
You can then use the password stanza from this file in my steps above.
Alternately if you have access to the mksysb from the problematic server, you can boot & mount /dev/hd4 as above, and then use this restore command to restore the passwd file.
I hope this helps!