In my opinion Tcl really straddles the line between a language and a library. I've heard it favorably compared to GLib. When used from C it feels a lot more like a library than other scripting languages do (Tcl 8 even more so than Jim due to a greater C API surface).

I agree with you on writing in another language but presumably you wouldn't be considering libpipeline anyway unless you had to write C. My default approach when I need C to access some API is to write an extension to an interpreter from which to access it. I leave embedding an interpreter in C for when you can't do that.