CL-USER> (ql:quickload :trivial-gray-streams)
To load "trivial-gray-streams":
  Load 1 ASDF system:
    trivial-gray-streams
; Loading "trivial-gray-streams"

(:TRIVIAL-GRAY-STREAMS)
CL-USER> (defclass transform-stream 
             (trivial-gray-streams:fundamental-character-output-stream)
           ((target :reader target
                    :initarg :target)))
#<STANDARD-CLASS COMMON-LISP-USER::TRANSFORM-STREAM>
CL-USER> (defmethod trivial-gray-streams:stream-write-char
             ((stream transform-stream) char)
           (write-char (if (char= char #\_) #\a char) (target stream)))
#<STANDARD-METHOD SB-GRAY:STREAM-WRITE-CHAR (TRANSFORM-STREAM T) {10018520D3}>
CL-USER> (defvar a (make-instance 'transform-stream :target *standard-output*))
A
CL-USER> (write-line "wibble_bar_quux")
wibble_bar_quux
"wibble_bar_quux"
CL-USER> (write-line "wibble_bar_quux" a)
wibbleabaraquux
"wibble_bar_quux"
CL-USER>