Reporting Services :: Aggregate Function On Large Value Type

We are constrained to use a model that stores records in a main table and various attributes of the record in a second table. Something like this: main record table CAR, attribute table CAR_DETAILS with CAR_DETAILS have key value pairs like "color" "blue" and "doors" "4".

For reports we need to flatten the one to many nature of this to get CAR and color and doors by using an aggregate function like: 

car_id, max(car_detail.value) filtered on car_detail.key = "color", max(car_detail.value) filtered on car_detail.key = "doors". 

This works on other tables but the car_detail table (shall we say) appears to store its values as blobs. In any case when we attempt to aggregate we get "The query uses an aggregate function on a large value type expression. Large value type expressions can not be aggregated."

Since we can't change the model, we would need to use another function to change this to a smaller string (or date, these are actually mostly date), but none of the very limited set of functions available seems to work to make this aggregation possible (and there is no "first" or anything else similar). 

The list of functions available in Report Model Queries can be found at [URL] .....