Indeed, I use my android tablet as a second screen for a Windows laptop and love it. No idea about headless displays though, I found the app superDisplay so jus

Indeed, I use my android tablet as a second screen for a Windows laptop and love it. No idea about headless displays though, I found the app superDisplay so just connect via usb cable and it all works like magic. I've actually got a little clip thing that attaches the tablet to the side of my laptop screen so I can dual screen easier. Setup also works for watching Netflix on a tablet while using the laptop on a bed or couch... Only downside is the tablet pulls charge from the laptop so you get less battery life especially if the tablet isn't fully charged to start.