All this talk of drag and drop and music has reminded me of some things: sunvox[1] and pure data[2]. Neither of them work exactly like pipes, but I think at the heart they are similar - the user creates nodes which do things and can drag the output of one into the input of another. Pure Data is much more of a full programming language (albeit media-focused) but they both capture what I think of when I think of GUI pipes quite will.

Which really, is just how analogue synthesizers and guitar pedals work; plugging wires into each other to layer effects. To be honest beyond music I'm struggling to find examples of how the piping metaphor can be found in real life (which is where I always look first when trying to find how to represent something on a computer).

[1]: https://warmplace.ru/soft/sunvox/
[2]: https://puredata.info/