Kdenlive is rather fragile and has a lot of bugs in handling filters (independent of the G’MIC stuff I am working on) and a lot of features of the plugin/filter APIs are not properly implemented. But I managed to get my generic G’MIC plugin running in there now with an input text field, although it looks horribly squished on the UI (see screenshot).
Automating parameters also works in Kdenlive now, with the simple blur here automated from 0 to 20 over the length of the video. Render time was surprisingly fast, almost realtime (~8s).
Here is the result: