Checking to see if anyone has made progress on this topic.
I'm working on building vCAC to a point where the Blueprints are very generic and all the customizations are user driven, however overriding the default machine prefix in the Blueprint is the one area that's causing me some problems. Ideally I'd like to have a drop down list of machine prefixes the user user can choose from. I could build a custom hostname based on pre-selected choices, but if I did this I don't know that I'd be able to leverage the machine prefix auto increments.