Hi @arbuthnot,
routing through an area is not an easy task, as the engine would have to create virual edges for a lot of places. There was a study on this a few years ago, but the resources needed to process this for global OSM data are not worth the benefit.
Here is an old blog entry about open space routing http://k1z.blog.uni-heidelberg.de/2017/06/24/introducing-labsopenrouteserviceorg-with-open-space-routing-and-the-places-poi-search-api/
I think OSRM can route around open spaces (on the borders), which is not quite routing through but might help you.
Best regards