The Splashtop Personal product is primarily a remote control product, but if you have two displays defined (even if you don't have the second monitor), it can show you either display on the ipad. If you view the secondary screen, but continue to control the PC from the PC itself, in effect it's a secondary display.
As for the extended-display-only project, it looks like they now have both wired and wireless versions of Xdisplay. Try the free versions (12 minute sessions at a time) and see if they work out for you. The wireless is supposed to work on Windows 10, but only gave me a mirrored screen. I didn't try the wired one yet.
At least someone's still working on a product that will do what you want.