Hope I can get an answer on this but could you also maybe answer one other questions.
If I make the modification as described, update the apache files etc. Would simply creating a new AMI based off the current image and creating a new ASG with that Image work as well?
Granted, this would mean that WP-Core updates would have to be performed outside updating the CF template but I would like to get a bit of feedback on this. The way I see it the flow would be
1) After installing WP Multisite, configuring it etc I would then create a new custom image
2) Create a new ASG with my Image
3) Scale up then down to replace the images (could also spin them down)
Updates to the image would basically be, scale down to 1 running instance, perform core updates (OS etc) create a new image and new ASG launch script and apply it.
Would that flow work?