I don’t know of an alternative plugin. OMV can support LDAP if you install it manually or when there is an plugin. I will have a look if I can do it in near future, but won’t promise it because the new UI for OMV6 has more priority.
So, this information is over 1 year old.
After LDAP is being listed as a Plugin under "Features" (https://www.openmediavault.org/features.html) I suppose this will be available within OMV6 again?
Less on the Server, this may be solved within extras and a LDAP Docker.
But being able to authenticate users would be helpful.
Edit: In my case, this does not need to be a posixAccount (with home-directory, uid, gid, etc.).
A inetOrgPerson and group membership would work (username, firstname, surname, mail). In my case focus is on granting access, the account itself can be managed by OMV and only takes the mentioned attributes from the LDAP server.
This probably reduces the integration efforts tremendously.