There isn’t any options in the software to restrict access to the administrator panel based on IP address. There’s already extensive protections for the admin interface, this was recently improved in release Pritunl v1.32.4512.98 with the additional NaCl request signatures.
It can be done without reverse proxies or load balancers by restricting specific paths to public access. A full list of the paths is available in pritunl-web/handlers/handlers.go.