Yes, so it seems that from the feedback on this thread I see here two things to means the same thing among different mappers:
A way tagged piste:type=downhill + piste:grooming=backcountry
A way tagged piste:type=downhill + piste:difficulty=freeride

Here is the cross-comparison:
piste:grooming value for pistes tagged piste:type=downhill + piste:difficulty=freeride

 count |    grooming     
-------+-----------------
   591 | 
   498 | backcountry

piste:difficulty values for piste:type=downhill + piste:grooming=backcountry

 count |  difficulty  
-------+--------------
   856 | advanced
   676 | intermediate
   546 | expert
   498 | freeride
   218 | easy
   137 | 
    21 | novice
    10 | extreme
    ... + noise

It also seems that most mappers take the pain to estimate a difficulty level for piste:type=downhill + piste:grooming=backcountry.