I can set Saved Asset searches to sort of include only offline assets, but I want to have a search that only includes online assets.
Unfortunately there’s no 100% effective way to do this. IIRC lastsynctime or whatever is the closest you get which isn’t very close. Even if you could Syncro’s Online/Offline status is notoriously unreliable. What are you trying to accomplish? Maybe skipping offline assets checkbox would help?
wrong forum, was meant to go to suggestions.
We will soon 
so stay tuned.