I have never used or developed for MLT, Kdenlive, Shotcut, etc., but the G’MIC C bridge library I developed some years ago (and which has since then been part of the regular G’MIC distribution) allows a developer to easily integrate G’MIC in other applications or plugin standards without any dependencies.
Well, I just downloaded Kdenlive and Shotcut and made a quick proof-of-concept filter using the frei0r SDK (which is one of the standards that MLT is based on). Within minutes I had G’MIC running in both Kdenlive and Shotcut as a plugin:
The plugin API seems a bit simple/limited, it does for example not appear to have a text input or button parameters, but in principle it works just like in the other hosts.