You may be able to set it via the prefs.
This is how I access it.
-(void)loadITunesPrefLibraryPath {
NSUserDefaults *userDefaults = [NSUserDefaults standardUserDefaults];
NSDictionary *userPref = [userDefaults persistentDomainForName:@"com.apple.iTunes"];
id dataBaseLoc = [userPref objectForKey:@"Database Location"];
NSLog(@"%s dataBaseLoc is:%@", __PRETTY_FUNCTION__, dataBaseLoc);
NSLog(@"%s dataBaseLoc class is:%@", __PRETTY_FUNCTION__, [dataBaseLoc class]);
NSData* dataBaseData = (NSData*)dataBaseLoc;
BOOL staleBook = NO;
NSError* bookError = nil;
NSURL* dataBaseURL = [NSURL URLByResolvingBookmarkData:dataBaseData options:NSURLBookmarkResolutionWithoutMounting relativeToURL:nil bookmarkDataIsStale:&staleBook error:&bookError];
self.libExtDBfile = dataBaseURL;
}
Once you get the userPrefs for iTunes.
And create a BookMarkData from URL.
You might be able to set it via
[userPref setObject:newDataBaseLoc forKey:@"Database Location"];
also see next answer for possible ITLibrary framework private API access