Templates API does proxy all templates kinds, but it requires URL to be in this form:

/unattended/:kind

In case of hostgroup provisioning, URLs are different:

/unattended/template/XXX/YYY

[lzap@lzapx foreman]$ curl -3 -H "Accept:application/json" -H "Content-Type: application/json" -d '{}' -k -X GET http://localhost:8448/unattended/template
Failed to retrieve template template for {"splat"=>[], "captures"=>["template"], "kind"=>"template"}: Error retrieving template for {:url=>"http://192.168.122.1:8448"} from 127.0.0.1: Net::HTTPNotFound

[lzap@lzapx foreman]$ curl -3 -H "Accept:application/json" -H "Content-Type: application/json" -d '{}' -k -X GET http://localhost:8448/unattended/template/clonetest/test-machines-rhel68

Requested url was not found

Workaround: Configure Satellite for direct access (non-proxied mode).