REP_TEXT_MUSTBEFREED looks like a bad idea, at lest for now: Errors can happen anywhere, and then sr_text is usually set to a string constant. If REP_TEXT_MUSTBEFREED is already set, slapd will then later try to free that constant.
To add this flag, first add result.c:rs_replace_text() or something similar, like rs_replace_entry(). Modify all code which modifies sr_text to use this function/macro, and encourage 3rd party code to do the same. Also look at code which copies a SlapReply. Each REP_*_MUSTBEFREED flag and its related data should be managed by a similar function/macro, really. (I tried to add a comment in Github, but that didn't seem to work, so mailing here instead.)
