Look, ma! I found in-memory predicate filtering.

llblgen-in-memory-predicate-filtering.cs

        public IEntityCollection2 FilterEntities(IEntityCollection2 collection, IRelationPredicateBucket filterBucket)
        {
            collection.DefaultView.Filter = filterBucket.PredicateExpression;  // only works for basic filter expressions
            return collection.DefaultView.ToEntityCollection();

/*
            // say goodbye to all this shitty code.
            var result = CreateEntityCollection(EntityType(collection));
            var filterField = "";
            object filterValue = null;

            if (filterBucket.PredicateExpression[0].Contents is PredicateExpression)
            {
                filterField = ((FieldCompareValuePredicate)((PredicateExpression)filterBucket.PredicateExpression[0].Contents)[0].Contents).FieldCore.Alias;
                filterValue = ((FieldCompareValuePredicate)((PredicateExpression)filterBucket.PredicateExpression[0].Contents)[0].Contents).Value;
            }
            else
            {
                filterField = ((FieldCompareRangePredicate)filterBucket.PredicateExpression[0].Contents).FieldCore.Alias;
                filterValue = ((FieldCompareRangePredicate)filterBucket.PredicateExpression[0].Contents).Values[0];
            }

            foreach (IEntity2 entity in collection)
            {
                if (entity.Fields[filterField].CurrentValue.Equals(filterValue))
                {
                    result.Add(entity);
                }
            }

            return result;
*/        }

:)