Although I would be interested to know if there's a fill exclusion layer. I haven't found anything on the docs

Yes, there are fill exclusion layers. In magic the layer is "fillblock". The layers should be in the list of GDS layer assignments. I try not to advertise them because if people rely on them too much then they can render the project unmanufacturable by having too low metal density.

Great, I will have a look. Thanks