Isnt there unicode string -> base string decoding function?
Or is the only option to recompile ECL without unicode?
But what encoding will you use? UTF-8? Some local encoding in the base string format? I really do not know what SBCL is doing about the buffer string there. It does not seem sensible that a low-level routine accepts a high-level object, a string. It is different with Common Lisp streams: there you have a sensible, well specified semantics for operating with large character sets.