Enable use of Tor only when using mobile data

I have a rather special fully Tor-ified network setup at home. When I'm connected to wifi, my device is behind Tor. Since connecting to Tor behind Tor is bad, when I want to use Briar, I have to get off wifi (or set the option in Briar to never connect to Tor, which isn't desired).

There's currently three options for the "Connect via Tor" setting: Never, Only when using Wi-Fi, and When using Wi-Fi or mobile data. I'd like a fourth option: Only when using mobile data. This way, I can always use Briar behind Tor.