I realize that, by now, there is a consensus that the limit is imposed by LDAP and not OPENQUERY.
However, in a previous job experience I was asked to get around the 8000-character limit imposed by OPENQUERY, and I think the same solution may apply here as well.
Here goes: I created a SQLCLR assembly that accepted my query as nvarchar(MAX) did the querying for me. I could pass in a query that was (practically) of any length, and it would stream records back to the database ad nauseum.
I think this workaround would bypass your LDAP limitation, but the truth is I never had anything related to LDAP come up at the time, so I don't know for certain.
- Brian from Chicago