First of all, read only functionality would be good enough for me. Especially if it can read from a database view which could be backed by my other tables. I often deal with user databases managed by custom software which we also want to let be consumed by something that speaks ldap, and it would be great not having to synchronize stuff, instead having a single source of truth.

So perhaps if it can read from a view with a schema defined by you, that would work? And perhaps for inserting/updating user/groups, you could require the database have a specific stored procedure present?

Initially I was thinking about allowing templated queries, where the administrator can specify the field names. For inserts, the database will just have to provide sensible defaults or let the insert go through a stored procedure that provides such. But doing everything using views & stored procedures might be simpler?