unfortunately, not possible from python ( cv2 ) atm.
while you can do it from c++ ,
string yml; // the whole schlepp in a string
FileStorage fs;
fs.open(yml,FileStorage::READ|FileStorage::MEMORY);
facereco->load(fs);
fs.release();
sad as it is, you can neither access FileStorage api, nor the FaceReco::load(FileStorage&) methods from python
(sidenote : at least you could resave them once from the facereco as yml.gz, to get the traffic down to 1/5 of the uncompressed xml)