Just to add my experience, I had given up on being able to use any of the Office 2007 Compatibility Pack on my Windows 98SE Office XP installation, until I read this thread, and I thought I'd give it a try.
I have the installation file for the base install, so I extracted the .msi installer file from it with Universal Extractor.
To my amazement, without any Orca modification, it ran and installed!
Word now opens docx files fine for reading, which is all I really wanted it for. I'm not interested in being able to save.
PowerPoint and Excel still don't open the newer formats, just throwing error messages, but Word was the important one for me, and I'm really pleased that it works!
I do have the installation file for SP3 of the Pack, but I guess that installing that is a no-no. When I run it it says it needs MS Installer version 3, and Universal Extractor doesn't seem to produce anything meaningful from it.