Have you looked at OpenBSD's ldapd(8) instead of openldap? It supports SASL PLAIN auth, according to the 6.3 man page.
I don't currently use SASL but otherwise have found the config of ldapd to be much simpler than slapd. Yes, last time I had set up OpenLDAP with SASL it was fragile and required a lot of fiddling to get it working. That was on Linux, about 10 years ago. Allan
