As you have already realized, vCAC blueprints are based on back-end templates. The case you are talking about is only possible if vCAC was deploying a machine when the user requested and then installed a OS etc which vCAC cannot do with vSphere blueprints.
But i know there is WinPE and Linux kickstart for physical provisioning. May be search on that line and see if you can achieve it. But as far as i know, that granularity and customization is not there in vCAC.
vCenter with just vCO could do that though. you can even build a portal using Orchestrator Web-views.
Abhilash B
LinkedIn : https://www.linkedin.com/in/abhilashhb/