I was wondering if there's a way/plug-in to sync only photos in collections based on metadata such as flagging and/or ratings? For instance, if I have a collection of 500 images, but only 50 are rated with 3 stars, only those 50 would be synced to my iPhone.
The easiest way I can see around this would be to create a smart collection with your filters then copy its contents to a new collection and sync the new collection. I realize that this is extra work you would probably like to avoid, but I don't believe there is any way around it at the current time.
I don't know if you are still searching but I had the same issue, I want many smart collections synced. I found Jeffrey Friedl's smart collection sync plugin to be the sollution.