Make it possible to access willMapDeserializedResponseBlock to modify the deserialized JSON response when you are using RKObjectManager.

gistfile1.m

@interface RKObjectManager (ModifyJSONRepsonse)

- (void)getObjectsAtPath:(NSString *)path
              parameters:(NSDictionary *)parameters
     modifyResponseBlock:(id (^)(id deserializedResponseBody))modifyResponseBlock
                 success:(void (^)(RKObjectRequestOperation *operation, RKMappingResult *mappingResult))success
                 failure:(void (^)(RKObjectRequestOperation *operation, NSError *error))failure;

- (void)getObjectsAtPathForRelationship:(NSString *)relationshipName
                               ofObject:(id)object
                             parameters:(NSDictionary *)parameters
                    modifyResponseBlock:(id (^)(id deserializedResponseBody))modifyResponseBlock
                                success:(void (^)(RKObjectRequestOperation *operation, RKMappingResult *mappingResult))success
                                failure:(void (^)(RKObjectRequestOperation *operation, NSError *error))failure;

- (void)getObjectsAtPathForRouteNamed:(NSString *)routeName
                               object:(id)object
                           parameters:(NSDictionary *)parameters
                  modifyResponseBlock:(id (^)(id deserializedResponseBody))modifyResponseBlock
                              success:(void (^)(RKObjectRequestOperation *operation, RKMappingResult *mappingResult))success
                              failure:(void (^)(RKObjectRequestOperation *operation, NSError *error))failure;

@end

@implementation RKObjectManager (ModifyJSONRepsonse)

- (void)getObjectsAtPath:(NSString *)path
              parameters:(NSDictionary *)parameters
     modifyResponseBlock:(id (^)(id deserializedResponseBody))modifyResponseBlock
                 success:(void (^)(RKObjectRequestOperation *operation, RKMappingResult *mappingResult))success
                 failure:(void (^)(RKObjectRequestOperation *operation, NSError *error))failure
{
    NSParameterAssert(path);
    RKObjectRequestOperation *operation = [self appropriateObjectRequestOperationWithObject:nil method:RKRequestMethodGET path:path parameters:parameters];
    [operation setCompletionBlockWithSuccess:success failure:failure];
    operation.willMapDeserializedResponseBlock = modifyResponseBlock;
    [self enqueueObjectRequestOperation:operation];
}

- (void)getObjectsAtPathForRelationship:(NSString *)relationshipName
                               ofObject:(id)object
                             parameters:(NSDictionary *)parameters
                    modifyResponseBlock:(id (^)(id deserializedResponseBody))modifyResponseBlock
                                success:(void (^)(RKObjectRequestOperation *operation, RKMappingResult *mappingResult))success
                                failure:(void (^)(RKObjectRequestOperation *operation, NSError *error))failure
{
    RKRoute *route = [self.router.routeSet routeForRelationship:relationshipName ofClass:[object class] method:RKRequestMethodGET];
    NSDictionary *interpolatedParameters = nil;
    NSURL *URL = [self URLWithRoute:route object:object interpolatedParameters:&interpolatedParameters];
    NSAssert(URL, @"Failed to generate URL for relationship named '%@' for object: %@", relationshipName, object);
    RKObjectRequestOperation *operation = [self appropriateObjectRequestOperationWithObject:nil method:RKRequestMethodGET path:[URL relativeString] parameters:parameters];
    operation.mappingMetadata = @{ @"routing": @{ @"parameters": interpolatedParameters, @"route": route } };
    [operation setCompletionBlockWithSuccess:success failure:failure];
    operation.willMapDeserializedResponseBlock = modifyResponseBlock;
    [self enqueueObjectRequestOperation:operation];
}

- (void)getObjectsAtPathForRouteNamed:(NSString *)routeName
                               object:(id)object
                           parameters:(NSDictionary *)parameters
                  modifyResponseBlock:(id (^)(id deserializedResponseBody))modifyResponseBlock
                              success:(void (^)(RKObjectRequestOperation *operation, RKMappingResult *mappingResult))success
                              failure:(void (^)(RKObjectRequestOperation *operation, NSError *error))failure
{
    NSParameterAssert(routeName);
    RKRoute *route = [self.router.routeSet routeForName:routeName];
    NSDictionary *interpolatedParameters = nil;
    NSURL *URL = [self URLWithRoute:route object:object interpolatedParameters:&interpolatedParameters];
    NSAssert(URL, @"No route found named '%@'", routeName);
    NSAssert(route.method == RKRequestMethodGET, @"Expected route named '%@' to specify a GET, but it does not", routeName);
    
    RKObjectRequestOperation *operation = [self appropriateObjectRequestOperationWithObject:nil method:RKRequestMethodGET path:[URL relativeString] parameters:parameters];
    operation.mappingMetadata = @{ @"routing": @{ @"parameters": interpolatedParameters, @"route": route } };
    [operation setCompletionBlockWithSuccess:success failure:failure];
    operation.willMapDeserializedResponseBlock = modifyResponseBlock;
    [self enqueueObjectRequestOperation:operation];
    
}

@end

Since URLWithRoute: method on RKObjectManager is private, it seems inaccessible to the category. How are you accessing it?