That solution kind of reminds me of an ARP request ("whohas 192.168.0.1")

I was trying to do something similar in terms of having my addon know if a toon existed (so when you would type a name in my white list, it would only allow you to put in valid toon names... to help avoid misspellings), but I found that if a toon was not online, I could get no info on them (they did not exist)

However, even if you have a valid toon name can't you just do UnitExists('name') because if they are offline, but exist, you get same result as if they don't exist at all.

From my reference materials, I would think UnitIsConnected(unitid|name) would work, but obviously, you've already tried it and found that it only works in party

This might be clunky, but what about making sure the target is either in guild or on friends list and have the player make sure the UI announces logon / logoff, then you could have your app watch the CHAT_MSG_SYSTEM event and then parse the messages to see if so-and-so has gone offline / come online?

Sorry if you've already tried this route too.