Is there any way to send a popup message from a Linux systen to a "random" laptop/tablet/mobile linked to my Wireless network ? For example, if I let my WLAN open and I see a not recognized computer connected to it, is there anyway to send to that device a message ?

On the other hand, if I am connected to someone else open network and they may or may not be aware that their network is open, can I send them a message warning that I am accessing their network?

Probably for a completely "random" device the answer should be no. But if we restrict to laptops with Win7 or Linux SO is there any service running by default on such systems that allows one to send such popup messages ?

PS: I have no practical motivation for this question. This is only a curiosity.