How to make a popup dialog appear on all devices connected to my WiFi, like with msg.exe or net send but cross platform?

I'd like to have a popup dialog box come up on all devices connected to my WiFi, say with the message "OK, you only get 15 mins of free WiFi". Is there anyways I开发者_JS百科 can broadcast this to all devices currently on my WiFi?

Not without running something on all devices.