I know that L2S is not designed to map custom/POCO types to L2S-entity types without the object initializer syntax. But is there a back-book way to achieve this so that i can project into my POCO with a parameterized constructor and not get stung when i try to apply an expression to that result?
You can't.
And if there is a "back-book way" I haven't found it (and I suspect I don't want to).