Matt Warren, who provides the most detailed HOWTO for creating your own LINQ IQueryable provider, has now published a dedicated toolkit on CodePlex:
IQToolkit is essential if you are building your own LINQ IQueryable provider. It contains common tools and source code you can apply to your own project.
In the toolkit you will find useful techniques for manipulating LINQ expression trees, implementing IQueryable providers, and a host of extensible components for building providers that target translation of LINQ expressions into SQL like languages.
Creating a LINQ provider is not a walk in the park, but at least if you have to, you have everything you need at hand.