Aruba Central does not support custom fields for harvesting data. Specifically things like voluntary email opt-ins, age verification, captcha, et al. We've requested enhancements to Cloud Guest numerous times and have been told it's in development, but that was 2 years ago.
I'd recommend you speak to your account team and log a request for enhancement to cloud guest. This is what my company has done as well. However, as of yet, we see no additional enhancement being done to Cloud Guest in any fashion, even after repeated requests. The most I was able to push for implementation on is basic in-line HTML so that we could use a hyperlink, or bold certain text.
Cloud Guest is lagging miles behind competitors, hopefully they do something about it soon.