Upon request here are some screenshots of some Nemo apps running in Plasma Active:
As you can see very much already works.
The patches/hacks I made, essentially, force full screen apps to run in “normal” mode instead of full screen and take care of sizing the app content correctly as shown in the screenshots. Without these patches full screen apps would cover the top bar and the app content would not scale correctly in some cases. Additionally, there had been some issue with the keyboard, which caused in some cases that the keyboard background was painted black instead of showing the app in the background.
There is also still room for improvement: e.g., apps forced to portrait mode are displayed sideways and in some cases the keyboard has slight issues like (in rare cases) still overpainting an app or that (also in only few cases) the update of the text field is not shown in “real time” but happens only after the keyboard had been closed.