I know you don't want to use the AVAssetImageGenerator but I've also researched this extensively and I believe the only solution currently is using the AVAssetImageGenerator. It's not that difficult as you say to get the right coordinate because you should be able to get the current time of your player. In my App the following code works perfectly:

-(UIImage *)getAVPlayerScreenshot 
{
    AVURLAsset *asset = (AVURLAsset *)self.playerItem.asset;
    AVAssetImageGenerator *imageGenerator = [[AVAssetImageGenerator alloc] initWithAsset:asset];
    imageGenerator.requestedTimeToleranceAfter = kCMTimeZero;
    imageGenerator.requestedTimeToleranceBefore = kCMTimeZero;
    CGImageRef thumb = [imageGenerator copyCGImageAtTime:self.playerItem.currentTime
                                              actualTime:NULL
                                                   error:NULL];
    UIImage *videoImage = [UIImage imageWithCGImage:thumb];
    CGImageRelease(thumb);
    return videoImage;
}