Credentials caching in OpenLDAP proxy
Solution 1:
I guess you already know about OpenLDAP Proxy Cache Engine since you are creating such server. Anyway, I would do this with the proxy cache engine; just set the cache TTL high enough so it can wait backend server reboots and other minor offline cases.
If you have multiple LDAP servers, you can also configure them in multi-master mode and put a load-balancer in front of them. That way you don't have to worry about a single LDAP server going down, everything continues to work smoothly automatically.