Anything you do in a client custom model that you pass along with the request you can also do with a server-side custom model you setup in your configuration file.
We did some work towards using OSM relations to set road attributes for roads located within the area of relations here: Add landuse encoded value based on closed-ring ways tagged with landuse=* by easbar · Pull Request #2765 · graphhopper/graphhopper · GitHub. But for the use case you are describing I think using the custom area feature, or the using the location index like karussell suggested would be ideal.