The guide has become a bit stale unfortunately because every time I try to edit the guide the FT editor breaks all of my links for mobile users within the post and then it becomes a delicate process to not break the links. If anyone knows if this has been fixed or has a workaround, please let me know and I'll make some updates.
For the original question, there were a few times where I was indeed able to prebook a spa treatment and cabana when flying in CW as a GGL, but I was later told this was either not allowed or no longer allowed. I'm not sure if it was a policy change or if I was just lucky the first few times.