Found out, yesterday, the game already does this when the service buildings are near a shelter and that means, when the shelter is activated, nearby services are automatically put on pause without turning it off. Unconfirmed, but I think the spending comes from public transit not being turned off during an evacuation. I'll look into this the next time I evacuate a whole town. See https://www.reddit.com/r/CitiesSkylinesModding/comments/1292796/mod_request_auto_off_services_for_disaster/?utm_source=share&utm_medium=web2x&context=3 for updates.