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iWork 09 is running fine on my installation of 10.11.1.

Suggestion:
1. Reboot to recovery partition
2. DISABLE system integrity protection
3. Boot to normal partition
4. Attempt iWork 09 installation.

That's how I did it...
 
Existing install launching fine for me here on 10.11.1 (except for the broken Share menu, but that was broken in Yosemite). I don't see how disabling SIP would have any effect. Pages doesn't use or modify kexts.
 
iWork 09 is running fine on my installation of 10.11.1.

Suggestion:
1. Reboot to recovery partition
2. DISABLE system integrity protection
3. Boot to normal partition
4. Attempt iWork 09 installation.

That's how I did it...

Not that understand these instructions but what if you have iWork '09 already installed? Will it NOT run if you upgrade to El Capitan, or what?
 
iWork 09 is running fine on my installation of 10.11.1.

Suggestion:
1. Reboot to recovery partition
2. DISABLE system integrity protection
3. Boot to normal partition
4. Attempt iWork 09 installation.

That's how I did it...
Could you actually not run iWork '09 prior to doing that? I would e very surprised if SIP had anything to say here.
 
Pages '09 has been working fine for me ever since the second beta of El Capitan. The issue is a non-issue.
 
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