There is, in /plugins/optional/, a carla.dll. I move it to /plugins/ and I can see what you see. But dragging it in to Song Editor crashes LMMS.
Perhaps, but judging from the page it links to right at the bottom it should.
https://github.com/LMMS/lmms/pull/5713
I can't figure out how to work it though. Could use some help here...