utf-8 worked everywhere and was backhandedly enforced by a lot of people using SBCL and utf-8 and sending reports to authors
I wasn't aware of this. Now the choice of UTF-8 is justified for me.
I did a ckeck of quicklisp systems. There are 263 lisp files in 107 systems which assume non-ASCII, and only 31 of them in 20 systems assume non-UTF-8:
./antik-20111105-git/physical-quantities/angle.lisp ./binary-types-20101006-git/example.lisp ./bknr-web-20111105-git/src/web/event-log.lisp ./cells-20101207-git/cells-test/build-sys.lisp ./cl-bibtex-20110110-cvs/cmp.lisp ./cl-pdf-20110418-svn/salza/compressor.lisp ./cl-png-0.6/._lisp-unit.lisp ./cl-typesetting-20110219-svn/examples/business-card.lisp ./clfswm-20120305-git/contrib/server/crypt.lisp ./clsql-20120305-git/examples/sqlite3/init-func/example.lisp ./linedit-20120208-git/terminfo.lisp ./lispbuilder-20110619-svn/lispbuilder-openrm/openrm/window.lisp ./lispbuilder-20110619-svn/lispbuilder-regex/regexp-test-suite.lisp ./mcclim-20110730-cvs/Examples/gadget-test-kr.lisp ./mcclim-20110730-cvs/Experimental/pixel-format.lisp ./mcclim-20110730-cvs/Tools/gilbert/clim-doc-convert.lisp ./mcclim-20120305-cvs/Examples/gadget-test-kr.lisp ./mcclim-20120305-cvs/Experimental/pixel-format.lisp ./mcclim-20120305-cvs/Tools/gilbert/clim-doc-convert.lisp ./metatilities-20101006-darcs/dev/contrib/mcl/appearance-mcl.lisp ./metatilities-20101006-darcs/dev/contrib/mcl/processes.lisp ./metatilities-20101006-darcs/dev/contrib/mcl/progress-indicator.lisp ./metatilities-20101006-darcs/dev/mcl/pop-up-menu.lisp ./montezuma-20120305-git/lucene-in-action/listing-2-1.lisp ./mtlisp-20110522-git/closstar.lisp ./mtlisp-20110522-git/ctrace.lisp ./mtlisp-20110522-git/mt-pkg.lisp ./phemlock-20120305-cvs/src/core/charmacs.lisp ./plain-odbc-20111105-svn/src/test/test-oracle.lisp ./regex-1/regexp-test-suite.lisp ./utils-kt-20101006-git/quad.lisp