How to give business unit write access on custom field?

I created one custom field on the Order entity. I want to make this field visible to everyone in the organization, but only give Write/Edit access to this (and only this) field to only a certain business group. I've tried creating a business rule or workflow but it seems I can only give the access based on the Owner of the record, which wouldn't work in my case, because the records can be owned by users not in that business group. I also don't want to give the Write access on the entire Order entity, so I'm guessing creating a new security role wouldn't work either. Can someone help?