How can I write text or put objects on a video with inverted colour as the background?

I did some research, but could find no answer.

I tried messing around with Chroma Key and colour invert on top of each other, but it did not work out.

the only way to make texts on Kdenlive are the titler tool.. or the "Dynamic Text" (and this one can just place a color or transparent color as a background) .. for now.. there are no "fast" ways to invert the background in a selected portion of clip...

You should use region effect, using a text clip (title) as the alpha mask.
But not sure region handling is back in refactored version (19.x), it has been in todo for a while...