This has been implemented with davdroid/3b89410b. Multiple SSIDs can now be entered (using comma as separator).
There is no escaping, so SSIDs containing , are not supported anymore.
It would be great if you make possibility to use app with same setting for internet/intranet. It might to be look like in connection settings add:
WiFi SSID: HOME_SSID
Why don’t you just use the correct (external) URL? It should be accessible locally, too.