Currently in the REST-API you can request transitive properties, e.g. when querying issue you could use @fields=priority.order (given that the priority table has an 'order' property). This currently only works for Link properties not for Multilink properties.
In our schema there is field 'pictures' in the user class linking to files with pictures of the user. You cannot currently request properties of pictures using @fields.

And, no, I don't have a patch yet :-)

Thanks for assigning to me!