REST API endpoint device/groups/{id}/devices

Just a word of caution -- we found long ago that using groups for taxonomy creates massive security problems if you also want to grant different users access to functional groups (e.g., SQL admins to SQL servers).  With RBAC as it is, if a device is in two or more such groups, you cannot give access that way without giving access to all groups the device is in (this is apparently not considered a bug).  There really needs to be an option to mark a group as a security group to avoid this.  In the meantime, we have moved from static groups almost entirely to dynamic groups.  Our biggest problem before was this one -- using location-based groups to organize devices and to avoid setting the location string many times.  Now we use custom propertysources to set a location property value to define a dynamic location group, and that group gets the location string. 

As far as your issue, I assume you could recurse to get the data, but definitely there should be a way to do this in one shot, just like inherited properties.