Matrix.org@gordon @duchesse LDAP is a bit of a mess in Matrix currently; we don't use it in the core team, and meanwhile the LDAP plugin for synapse is a bit simplistic. However: https://github.com/matrix-org/matrix-synapse-ldap3/blob/master/ldap_auth_provider.py#L94-L140 is where you'd put in a check to let non-LDAP usernames register. Alternatively try asking on #matrix-identity:matrix.org; it's possible mxisd could also be a solution.
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