Are these supported under Kdenlive?
If yes, how do you use them?

2 input mixers are exposed as transitions; however, many of them are not that interesting alone strictly as transitions.

I am working on some Frei0r sources (test pattern generators), and use Kdenlive for testing.
Now I declare them to be filters, and simply ignore the input frames.

But this way, I must have an underlying video (I just make a black color clip), to which I can apply them (can't just put them on an empty track).

So I was wondering, is there a way to put a frei0r source onto an empty track?

The two generators (sources) that are currently supported (noise, countdown) are provided as a "sample plugin".
I think it should not be to hard to add full "frei0r.sources" support based on the plugin's source and the way effects and transitions are treated.

On that note, please keep in mind that frei0r sources are already available in MLT as MLT producers. So, really, you just need a Kdenlive change or plugin to expose MLT producers that begin with "frei0r.".