Looking at the tables I don't see how you could do it directly, but I'm thinking that there may be a workaround: Maybe you could make a policy that has modifiers that only work for a particular Civ. REQUIREMENT_CIV_TYPE_MATCHES might do the job for you, but it comes with a huge caveat – None of the existing code uses it, so you'll just have to hope that the code to make it work exists (and of course that I'm not talking complete rubbish, which is possible as I've only been modding Civ VI for a few weeks).