But I think it would be better to keep the indentation code in Emacs than to make a fork.
I don't use Emacs but XEmacs so I'm in favor of keeping the indentation code in the Slime CVS and to make it the "official" one (or at least the cutting edge one ;-) so that people can contribute to it.
BTW the latest modification for find-coding-system does not work on XEmacs 21.5.18 Win32 (I had to force it to binary).
Marc