From what I've read/studied (though I am not a lawyer), fair use is an affirmative defense against copyright infringement in the US.

I'll have to think more about the readline interface bit you raise. My gut instinct suggests that's not quite right, but my rational brain isn't able to label/ID/argue why, so your point has some merit. I'll need to think about it some (so thanks for that [seriously]!)

Maybe there's another line input library with a compatible binary interface and MIT licensed and clisp is written to interface to that library. That would seem like it might fly.