Quite honestly, it has always made me smile to have a mother-in-law, sister-in-law etc as I don't feel they're family I'm just as happy calling MIL 'my husband's name's' mother as that's who she is. Your daughter's mother-in-law is in fact your son-in-law's mother.
Here in France, MILs are 'belle-m�res' literally 'beautiful mothers' some of us have a tendancy of calling them 'moche-m�res' between us meaning ugly mothers!!!
If I were to choose a name as you question, I'd call them co-parents.