Here is a test case for SBCL and CLISP. (CLISP part needs my last patch applied.) This ASDF system is supposed to compile and load without warnings or errors.
That patch makes CLISP behave like this when a charset cannot be found:
Error while trying to load definition for system external-format-test from pathname /tmp/external-format-test/external-format-test.asd: INTERN("SHIFT_JIS"): #<PACKAGE CHARSET> is locked
If I replace #'intern with #'find-symbol in it the message is:
OPEN: Illegal :EXTERNAL-FORMAT argument :SHIFT_JIS
with a useful restart USE-VALUE. I think the latter is somewhat better, hence updated patch.
I'd also like to express concern that this change has to be properly announced because it will affect those who use ASDF for their local projects with non UTF-8 compliant encoding and previously assumed that other ASDF systems fit into ASCII.