If I am not entirely mistaken, the `fossil unversioned' command accepts "-R | --repository" as an option, but this isn't documented in the help output.  This is by virtue of db_find_and_open_repository which handles the options in question.

Index: src/unversioned.c
==================================================================
--- src/unversioned.c
+++ src/unversioned.c
@@ -267,10 +267,11 @@
 **
 ** Options:
 **
 **   --mtime TIMESTAMP       Use TIMESTAMP instead of "now" for the "add",
 **                           "edit", "remove", and "touch" subcommands.
+**   -R|--repository FILE    Extract info from repository FILE.
 */
 void unversioned_cmd(void){
   const char *zCmd;
   int nCmd;
   const char *zMtime = find_option("mtime", 0, 1);