What you put in the Map is what is being indexed and what you put in TransformResults is what you get back from a successful query. Therefore you can never query the tags unless you add them in the map.
What you could do is to use LoadDocument
in the map. It is a fairly new feature that was made available in 2.0. The docs are here: http://ravendb.net/docs/2.0/client-api/querying/static-indexes/indexing-related-documents
This is what the map could look like in your case perhaps:
from product in docs.Products
select new {
product.Id,
product.ManufacturerId,
Tags = LoadDocument("manufacturers/"+ product.ManufacturerId).Tags
}
The TransformResults you are using could remain the same.
A tip is to use the management studio and look what is indexed. You can do so in Indexes -> [your index] -> Query and then check the Index entries checkbox from the Query Options drop down.