It seems there is potential bug in setxkbmap. Look at applyRules function:

        if (svSrc[RULES_NDX])
            rfName = svValue[RULES_NDX];
        else
            rfName = DFLT_XKB_RULES_FILE;
##### rfName is either specified name of rules file, or default name. 
##### It seems further rfName should be used as name of rules file.
        if (rfName[0] == '/')
        {
            rules = XkbRF_Load(rfName, svValue[LOCALE_NDX], True, True);
##### That's ok.
        }
        else
        {
            /* try to load rules files from all include paths until the first
             * we succeed with */
            for (i = 0; (i < numInclPath) && (!rules); i++)
            {
                if ((strlen(inclPath[i]) + strlen(rfName) + 8) > PATH_MAX)
##### rfName is used for calculating length of path.
                {
                    VMSG2(0, "Path too long (%s/rules/%s). Ignored.\n",
                          inclPath[i], rfName);
##### rfName is used in warning message.
                    continue;
                }
                sprintf(buf, "%s/rules/%s", inclPath[i], svValue[RULES_NDX]);
##### Oops! svValue[RULES_NDX] is used. I guess it should be rfName instead.
                rules = XkbRF_Load(buf, svValue[LOCALE_NDX], True, True);
            }
        }
        if (!rules)
        {
            ERR1("Couldn't find rules file (%s) \n", svValue[RULES_NDX]);
#### Oops again. It seems rfName should be used there, not svValue[RULES_DNX].
            return False;
        }

If you agree it should be fixed, I can prepare a patch.