Locking realm access to a specific huntgroup

Hello, I have different NAS and each type of NAS is grouped together in a huntgroup. I need to make an addition to my radius setup to proxy requests with a certain realm to a specified server. Proxying is already working but I want to lock the users using that specific realm to a specific huntgroup so that its only possible for them to login on predefined NASgroup. I don't have access to the remote proxy server. Using attr_filter isn't an option since this only filters replys. Anyone knows how to do this? Thx in advance and kind regards, Jonathan