Hey there,
Brian here from the support engineering team, just wanted to provide some insight on this issue for visibility.
Unfortunately, there aren’t any accessible methods that could be used to retrieve custom properties from profiles through our Klaviyo JS. These calls are generally used to POST information to the user profile and retrieval from the front end would create somewhat of a security risk. Typically as it relates to customer information, most of our calls would require a request containing your private api key in which you’d generally have to pass through on your backend or through middleware.
I know you mentioned that our API isn’t really a feasible option for you, but we'd really only be able to make these calls via our API either from your backend or another REST api to serve up these calls to your frontend.
We may potentially have some alternatives in the pipeline with our v3 API however nothing is set in stone yet for the general release. In the interim, if you do reconsider using our API, your best bet would be our get profile endpoint:
https://developers.klaviyo.com/en/reference/get-profile
Thanks for your patience and understanding; I’ll be sure to pass this along as a potential blocker for users looking to make use of custom properties client side.