Is there a way to insert a value into an id column, similarly to what I would do if i was using SET IDENTITY_INSERT ON in a stored procedure? I've tried the standard:

var person = new Person();
person.AddNew();
person.id = 250;
person.name = "Bob Smith";
person.Save();

But when i do this, it creates its own identity, rather than using the one i gave it.

No, out-of-the-box, EntitySpaces does not allow setting identity PKs. But, you an create your own SP, add a method to call it in your custom class, then use that in your code, instead of Save().