ViGEm works great on my Linx Vision 8 Windows 10 tablet with the XBOX360 controller dock, but for some reason the remoteplay app (for PS4) itself won't use the touchscreen as the touchpad, so I lose out on some functionality. I don't suppose you could add an option to take mouse events and use them as the touchpad?
Woha, that thing looks fancy 
Currently the DS4 emulated by ViGEm lacks (API-) support for feeding its touch pad, it's on the road map but due to little interest - partially from my assessment - it's not done. Where the input comes from (mouse, real touch screen) is then a responsibility of a feeder application which bridges the gap and proxies the input data between said input device and a ViGEm controller.
UCR might be able to do this (As and when Nefarius adds the API end-points for outputting to DS4 touchpad).
We have an Interception provider for keyboard and mouse, which supports absolute mouse input. This may work with the touchscreen, not sure if you will get multi-touch, but I don't think you want that?