Prefetch paths are for fetching graphs in 1 go. filtering on related entities is something else: you want to fetch a single set, and limit that set on related entities.
Say I want to fetch all orders, from customers which are from Germany:
var q = from o in metaData.Order
where o.Customer.Country=="Germany"
select o;
here, I navigate over 'Customer', and the provider knows what relation that is and does the join for you
With this info, could you formulate the query yourself, or did you run into a wall while trying?