This exists. Every reletively new Android product (>=4.1) has "Mobile hotspots" under "Data usage".

> Network bandwidth management: New API provides ability to detect metered networks, including tethering to a mobile hotspot.

Mark the WiFi access points you want to be low data usage and you are good to go. Software that plays nice will use the "isActiveNetworkMetered()" to check if they should download or not.