leftside I have been experimenting with faces data stored in embedded XMP for some time and it worked fine (I am talking about import of markup to Tonfotos database), but I would not say that I am very confident it will work in all cases, as I did my tests just on handful of samples. This area still requires more testing, and as you said, there is no standard and result vary depending on application that has been used to write this markup.
I would encourage you to try it out yourself on small sample set. Just copy few samples into separate directory and add this directory into Tonfotos index. Since this is just a few sample, indexing should not take long. In case you encounter any issues, I would be happy to take a look at your samples and adjust Tonfotos algorithms to better support Windows Live Photos.